1 | =head1 NAME
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2 | X<syntax>
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3 |
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4 | perlsyn - Perl syntax
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5 |
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6 | =head1 DESCRIPTION
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7 |
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8 | A Perl program consists of a sequence of declarations and statements
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9 | which run from the top to the bottom. Loops, subroutines and other
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10 | control structures allow you to jump around within the code.
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11 |
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12 | Perl is a B<free-form> language, you can format and indent it however
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13 | you like. Whitespace mostly serves to separate tokens, unlike
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14 | languages like Python where it is an important part of the syntax.
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15 |
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16 | Many of Perl's syntactic elements are B<optional>. Rather than
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17 | requiring you to put parentheses around every function call and
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18 | declare every variable, you can often leave such explicit elements off
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19 | and Perl will figure out what you meant. This is known as B<Do What I
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20 | Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
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21 | code in a style with which they are comfortable.
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22 |
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23 | Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
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24 | Bourne Shell, Smalltalk, Lisp and even English. Other
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25 | languages have borrowed syntax from Perl, particularly its regular
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26 | expression extensions. So if you have programmed in another language
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27 | you will see familiar pieces in Perl. They often work the same, but
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28 | see L<perltrap> for information about how they differ.
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29 |
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30 | =head2 Declarations
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31 | X<declaration> X<undef> X<undefined> X<uninitialized>
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32 |
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33 | The only things you need to declare in Perl are report formats and
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34 | subroutines (and sometimes not even subroutines). A variable holds
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35 | the undefined value (C<undef>) until it has been assigned a defined
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36 | value, which is anything other than C<undef>. When used as a number,
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37 | C<undef> is treated as C<0>; when used as a string, it is treated as
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38 | the empty string, C<"">; and when used as a reference that isn't being
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39 | assigned to, it is treated as an error. If you enable warnings,
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40 | you'll be notified of an uninitialized value whenever you treat
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41 | C<undef> as a string or a number. Well, usually. Boolean contexts,
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42 | such as:
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43 |
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44 | my $a;
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45 | if ($a) {}
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46 |
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47 | are exempt from warnings (because they care about truth rather than
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48 | definedness). Operators such as C<++>, C<-->, C<+=>,
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49 | C<-=>, and C<.=>, that operate on undefined left values such as:
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50 |
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51 | my $a;
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52 | $a++;
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53 |
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54 | are also always exempt from such warnings.
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55 |
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56 | A declaration can be put anywhere a statement can, but has no effect on
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57 | the execution of the primary sequence of statements--declarations all
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58 | take effect at compile time. Typically all the declarations are put at
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59 | the beginning or the end of the script. However, if you're using
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60 | lexically-scoped private variables created with C<my()>, you'll
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61 | have to make sure
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62 | your format or subroutine definition is within the same block scope
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63 | as the my if you expect to be able to access those private variables.
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64 |
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65 | Declaring a subroutine allows a subroutine name to be used as if it were a
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66 | list operator from that point forward in the program. You can declare a
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67 | subroutine without defining it by saying C<sub name>, thus:
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68 | X<subroutine, declaration>
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69 |
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70 | sub myname;
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71 | $me = myname $0 or die "can't get myname";
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72 |
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73 | Note that myname() functions as a list operator, not as a unary operator;
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74 | so be careful to use C<or> instead of C<||> in this case. However, if
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75 | you were to declare the subroutine as C<sub myname ($)>, then
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76 | C<myname> would function as a unary operator, so either C<or> or
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77 | C<||> would work.
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78 |
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79 | Subroutines declarations can also be loaded up with the C<require> statement
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80 | or both loaded and imported into your namespace with a C<use> statement.
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81 | See L<perlmod> for details on this.
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82 |
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83 | A statement sequence may contain declarations of lexically-scoped
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84 | variables, but apart from declaring a variable name, the declaration acts
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85 | like an ordinary statement, and is elaborated within the sequence of
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86 | statements as if it were an ordinary statement. That means it actually
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87 | has both compile-time and run-time effects.
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88 |
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89 | =head2 Comments
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90 | X<comment> X<#>
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91 |
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92 | Text from a C<"#"> character until the end of the line is a comment,
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93 | and is ignored. Exceptions include C<"#"> inside a string or regular
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94 | expression.
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95 |
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96 | =head2 Simple Statements
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97 | X<statement> X<semicolon> X<expression> X<;>
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98 |
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99 | The only kind of simple statement is an expression evaluated for its
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100 | side effects. Every simple statement must be terminated with a
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101 | semicolon, unless it is the final statement in a block, in which case
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102 | the semicolon is optional. (A semicolon is still encouraged if the
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103 | block takes up more than one line, because you may eventually add
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104 | another line.) Note that there are some operators like C<eval {}> and
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105 | C<do {}> that look like compound statements, but aren't (they're just
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106 | TERMs in an expression), and thus need an explicit termination if used
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107 | as the last item in a statement.
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108 |
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109 | =head2 Truth and Falsehood
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110 | X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
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111 |
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112 | The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
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113 | C<undef> are all false in a boolean context. All other values are true.
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114 | Negation of a true value by C<!> or C<not> returns a special false value.
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115 | When evaluated as a string it is treated as C<''>, but as a number, it
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116 | is treated as 0.
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117 |
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118 | =head2 Statement Modifiers
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119 | X<statement modifier> X<modifier> X<if> X<unless> X<while>
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120 | X<until> X<foreach> X<for>
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121 |
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122 | Any simple statement may optionally be followed by a I<SINGLE> modifier,
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123 | just before the terminating semicolon (or block ending). The possible
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124 | modifiers are:
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125 |
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126 | if EXPR
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127 | unless EXPR
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128 | while EXPR
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129 | until EXPR
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130 | foreach LIST
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131 |
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132 | The C<EXPR> following the modifier is referred to as the "condition".
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133 | Its truth or falsehood determines how the modifier will behave.
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134 |
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135 | C<if> executes the statement once I<if> and only if the condition is
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136 | true. C<unless> is the opposite, it executes the statement I<unless>
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137 | the condition is true (i.e., if the condition is false).
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138 |
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139 | print "Basset hounds got long ears" if length $ear >= 10;
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140 | go_outside() and play() unless $is_raining;
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141 |
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142 | The C<foreach> modifier is an iterator: it executes the statement once
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143 | for each item in the LIST (with C<$_> aliased to each item in turn).
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144 |
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145 | print "Hello $_!\n" foreach qw(world Dolly nurse);
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146 |
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147 | C<while> repeats the statement I<while> the condition is true.
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148 | C<until> does the opposite, it repeats the statement I<until> the
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149 | condition is true (or while the condition is false):
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150 |
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151 | # Both of these count from 0 to 10.
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152 | print $i++ while $i <= 10;
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153 | print $j++ until $j > 10;
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154 |
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155 | The C<while> and C<until> modifiers have the usual "C<while> loop"
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156 | semantics (conditional evaluated first), except when applied to a
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157 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
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158 | which case the block executes once before the conditional is
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159 | evaluated. This is so that you can write loops like:
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160 |
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161 | do {
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162 | $line = <STDIN>;
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163 | ...
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164 | } until $line eq ".\n";
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165 |
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166 | See L<perlfunc/do>. Note also that the loop control statements described
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167 | later will I<NOT> work in this construct, because modifiers don't take
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168 | loop labels. Sorry. You can always put another block inside of it
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169 | (for C<next>) or around it (for C<last>) to do that sort of thing.
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170 | For C<next>, just double the braces:
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171 | X<next> X<last> X<redo>
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172 |
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173 | do {{
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174 | next if $x == $y;
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175 | # do something here
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176 | }} until $x++ > $z;
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177 |
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178 | For C<last>, you have to be more elaborate:
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179 | X<last>
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180 |
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181 | LOOP: {
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182 | do {
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183 | last if $x = $y**2;
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184 | # do something here
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185 | } while $x++ <= $z;
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186 | }
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187 |
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188 | B<NOTE:> The behaviour of a C<my> statement modified with a statement
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189 | modifier conditional or loop construct (e.g. C<my $x if ...>) is
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190 | B<undefined>. The value of the C<my> variable may be C<undef>, any
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191 | previously assigned value, or possibly anything else. Don't rely on
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192 | it. Future versions of perl might do something different from the
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193 | version of perl you try it out on. Here be dragons.
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194 | X<my>
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195 |
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196 | =head2 Compound Statements
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197 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
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198 | X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue>
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199 |
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200 | In Perl, a sequence of statements that defines a scope is called a block.
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201 | Sometimes a block is delimited by the file containing it (in the case
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202 | of a required file, or the program as a whole), and sometimes a block
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203 | is delimited by the extent of a string (in the case of an eval).
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204 |
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205 | But generally, a block is delimited by curly brackets, also known as braces.
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206 | We will call this syntactic construct a BLOCK.
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207 |
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208 | The following compound statements may be used to control flow:
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209 |
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210 | if (EXPR) BLOCK
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211 | if (EXPR) BLOCK else BLOCK
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212 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
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213 | LABEL while (EXPR) BLOCK
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214 | LABEL while (EXPR) BLOCK continue BLOCK
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215 | LABEL until (EXPR) BLOCK
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216 | LABEL until (EXPR) BLOCK continue BLOCK
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217 | LABEL for (EXPR; EXPR; EXPR) BLOCK
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218 | LABEL foreach VAR (LIST) BLOCK
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219 | LABEL foreach VAR (LIST) BLOCK continue BLOCK
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220 | LABEL BLOCK continue BLOCK
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221 |
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222 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
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223 | not statements. This means that the curly brackets are I<required>--no
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224 | dangling statements allowed. If you want to write conditionals without
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225 | curly brackets there are several other ways to do it. The following
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226 | all do the same thing:
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227 |
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228 | if (!open(FOO)) { die "Can't open $FOO: $!"; }
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229 | die "Can't open $FOO: $!" unless open(FOO);
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230 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
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231 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
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232 | # a bit exotic, that last one
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233 |
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234 | The C<if> statement is straightforward. Because BLOCKs are always
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235 | bounded by curly brackets, there is never any ambiguity about which
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236 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
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237 | the sense of the test is reversed.
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238 |
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239 | The C<while> statement executes the block as long as the expression is
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240 | true (does not evaluate to the null string C<""> or C<0> or C<"0">).
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241 | The C<until> statement executes the block as long as the expression is
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242 | false.
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243 | The LABEL is optional, and if present, consists of an identifier followed
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244 | by a colon. The LABEL identifies the loop for the loop control
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245 | statements C<next>, C<last>, and C<redo>.
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246 | If the LABEL is omitted, the loop control statement
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247 | refers to the innermost enclosing loop. This may include dynamically
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248 | looking back your call-stack at run time to find the LABEL. Such
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249 | desperate behavior triggers a warning if you use the C<use warnings>
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250 | pragma or the B<-w> flag.
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251 |
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252 | If there is a C<continue> BLOCK, it is always executed just before the
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253 | conditional is about to be evaluated again. Thus it can be used to
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254 | increment a loop variable, even when the loop has been continued via
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255 | the C<next> statement.
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256 |
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257 | =head2 Loop Control
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258 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
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259 |
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260 | The C<next> command starts the next iteration of the loop:
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261 |
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262 | LINE: while (<STDIN>) {
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263 | next LINE if /^#/; # discard comments
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264 | ...
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265 | }
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266 |
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267 | The C<last> command immediately exits the loop in question. The
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268 | C<continue> block, if any, is not executed:
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269 |
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270 | LINE: while (<STDIN>) {
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271 | last LINE if /^$/; # exit when done with header
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272 | ...
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273 | }
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274 |
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275 | The C<redo> command restarts the loop block without evaluating the
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276 | conditional again. The C<continue> block, if any, is I<not> executed.
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277 | This command is normally used by programs that want to lie to themselves
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278 | about what was just input.
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279 |
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280 | For example, when processing a file like F</etc/termcap>.
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281 | If your input lines might end in backslashes to indicate continuation, you
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282 | want to skip ahead and get the next record.
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283 |
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284 | while (<>) {
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285 | chomp;
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286 | if (s/\\$//) {
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287 | $_ .= <>;
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288 | redo unless eof();
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289 | }
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290 | # now process $_
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291 | }
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292 |
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293 | which is Perl short-hand for the more explicitly written version:
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294 |
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295 | LINE: while (defined($line = <ARGV>)) {
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296 | chomp($line);
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297 | if ($line =~ s/\\$//) {
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298 | $line .= <ARGV>;
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299 | redo LINE unless eof(); # not eof(ARGV)!
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300 | }
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301 | # now process $line
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302 | }
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303 |
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304 | Note that if there were a C<continue> block on the above code, it would
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305 | get executed only on lines discarded by the regex (since redo skips the
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306 | continue block). A continue block is often used to reset line counters
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307 | or C<?pat?> one-time matches:
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308 |
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309 | # inspired by :1,$g/fred/s//WILMA/
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310 | while (<>) {
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311 | ?(fred)? && s//WILMA $1 WILMA/;
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312 | ?(barney)? && s//BETTY $1 BETTY/;
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313 | ?(homer)? && s//MARGE $1 MARGE/;
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314 | } continue {
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315 | print "$ARGV $.: $_";
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316 | close ARGV if eof(); # reset $.
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317 | reset if eof(); # reset ?pat?
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318 | }
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319 |
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320 | If the word C<while> is replaced by the word C<until>, the sense of the
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321 | test is reversed, but the conditional is still tested before the first
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322 | iteration.
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323 |
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324 | The loop control statements don't work in an C<if> or C<unless>, since
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325 | they aren't loops. You can double the braces to make them such, though.
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326 |
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327 | if (/pattern/) {{
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328 | last if /fred/;
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329 | next if /barney/; # same effect as "last", but doesn't document as well
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330 | # do something here
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331 | }}
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332 |
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333 | This is caused by the fact that a block by itself acts as a loop that
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334 | executes once, see L<"Basic BLOCKs and Switch Statements">.
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335 |
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336 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
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337 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
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338 |
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339 | =head2 For Loops
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340 | X<for> X<foreach>
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341 |
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342 | Perl's C-style C<for> loop works like the corresponding C<while> loop;
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343 | that means that this:
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344 |
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345 | for ($i = 1; $i < 10; $i++) {
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346 | ...
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347 | }
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348 |
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349 | is the same as this:
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350 |
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351 | $i = 1;
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352 | while ($i < 10) {
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353 | ...
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354 | } continue {
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355 | $i++;
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356 | }
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357 |
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358 | There is one minor difference: if variables are declared with C<my>
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359 | in the initialization section of the C<for>, the lexical scope of
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360 | those variables is exactly the C<for> loop (the body of the loop
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361 | and the control sections).
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362 | X<my>
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363 |
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364 | Besides the normal array index looping, C<for> can lend itself
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365 | to many other interesting applications. Here's one that avoids the
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366 | problem you get into if you explicitly test for end-of-file on
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367 | an interactive file descriptor causing your program to appear to
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368 | hang.
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369 | X<eof> X<end-of-file> X<end of file>
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370 |
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371 | $on_a_tty = -t STDIN && -t STDOUT;
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372 | sub prompt { print "yes? " if $on_a_tty }
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373 | for ( prompt(); <STDIN>; prompt() ) {
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374 | # do something
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375 | }
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376 |
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377 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the
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378 | conditional of a C<for> loop is shorthand for the following. This
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379 | behaviour is the same as a C<while> loop conditional.
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380 | X<readline> X<< <> >>
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381 |
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382 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
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383 | # do something
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384 | }
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385 |
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386 | =head2 Foreach Loops
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387 | X<for> X<foreach>
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388 |
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389 | The C<foreach> loop iterates over a normal list value and sets the
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390 | variable VAR to be each element of the list in turn. If the variable
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391 | is preceded with the keyword C<my>, then it is lexically scoped, and
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392 | is therefore visible only within the loop. Otherwise, the variable is
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393 | implicitly local to the loop and regains its former value upon exiting
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394 | the loop. If the variable was previously declared with C<my>, it uses
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395 | that variable instead of the global one, but it's still localized to
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396 | the loop. This implicit localisation occurs I<only> in a C<foreach>
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397 | loop.
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398 | X<my> X<local>
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399 |
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400 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so
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401 | you can use C<foreach> for readability or C<for> for brevity. (Or because
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402 | the Bourne shell is more familiar to you than I<csh>, so writing C<for>
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403 | comes more naturally.) If VAR is omitted, C<$_> is set to each value.
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404 | X<$_>
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405 |
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406 | If any element of LIST is an lvalue, you can modify it by modifying
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407 | VAR inside the loop. Conversely, if any element of LIST is NOT an
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408 | lvalue, any attempt to modify that element will fail. In other words,
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409 | the C<foreach> loop index variable is an implicit alias for each item
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410 | in the list that you're looping over.
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411 | X<alias>
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412 |
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413 | If any part of LIST is an array, C<foreach> will get very confused if
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414 | you add or remove elements within the loop body, for example with
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415 | C<splice>. So don't do that.
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416 | X<splice>
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417 |
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418 | C<foreach> probably won't do what you expect if VAR is a tied or other
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419 | special variable. Don't do that either.
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420 |
|
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421 | Examples:
|
---|
422 |
|
---|
423 | for (@ary) { s/foo/bar/ }
|
---|
424 |
|
---|
425 | for my $elem (@elements) {
|
---|
426 | $elem *= 2;
|
---|
427 | }
|
---|
428 |
|
---|
429 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
|
---|
430 | print $count, "\n"; sleep(1);
|
---|
431 | }
|
---|
432 |
|
---|
433 | for (1..15) { print "Merry Christmas\n"; }
|
---|
434 |
|
---|
435 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
|
---|
436 | print "Item: $item\n";
|
---|
437 | }
|
---|
438 |
|
---|
439 | Here's how a C programmer might code up a particular algorithm in Perl:
|
---|
440 |
|
---|
441 | for (my $i = 0; $i < @ary1; $i++) {
|
---|
442 | for (my $j = 0; $j < @ary2; $j++) {
|
---|
443 | if ($ary1[$i] > $ary2[$j]) {
|
---|
444 | last; # can't go to outer :-(
|
---|
445 | }
|
---|
446 | $ary1[$i] += $ary2[$j];
|
---|
447 | }
|
---|
448 | # this is where that last takes me
|
---|
449 | }
|
---|
450 |
|
---|
451 | Whereas here's how a Perl programmer more comfortable with the idiom might
|
---|
452 | do it:
|
---|
453 |
|
---|
454 | OUTER: for my $wid (@ary1) {
|
---|
455 | INNER: for my $jet (@ary2) {
|
---|
456 | next OUTER if $wid > $jet;
|
---|
457 | $wid += $jet;
|
---|
458 | }
|
---|
459 | }
|
---|
460 |
|
---|
461 | See how much easier this is? It's cleaner, safer, and faster. It's
|
---|
462 | cleaner because it's less noisy. It's safer because if code gets added
|
---|
463 | between the inner and outer loops later on, the new code won't be
|
---|
464 | accidentally executed. The C<next> explicitly iterates the other loop
|
---|
465 | rather than merely terminating the inner one. And it's faster because
|
---|
466 | Perl executes a C<foreach> statement more rapidly than it would the
|
---|
467 | equivalent C<for> loop.
|
---|
468 |
|
---|
469 | =head2 Basic BLOCKs and Switch Statements
|
---|
470 | X<switch> X<block> X<case>
|
---|
471 |
|
---|
472 | A BLOCK by itself (labeled or not) is semantically equivalent to a
|
---|
473 | loop that executes once. Thus you can use any of the loop control
|
---|
474 | statements in it to leave or restart the block. (Note that this is
|
---|
475 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
|
---|
476 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
|
---|
477 | block is optional.
|
---|
478 |
|
---|
479 | The BLOCK construct is particularly nice for doing case
|
---|
480 | structures.
|
---|
481 |
|
---|
482 | SWITCH: {
|
---|
483 | if (/^abc/) { $abc = 1; last SWITCH; }
|
---|
484 | if (/^def/) { $def = 1; last SWITCH; }
|
---|
485 | if (/^xyz/) { $xyz = 1; last SWITCH; }
|
---|
486 | $nothing = 1;
|
---|
487 | }
|
---|
488 |
|
---|
489 | There is no official C<switch> statement in Perl, because there are
|
---|
490 | already several ways to write the equivalent.
|
---|
491 |
|
---|
492 | However, starting from Perl 5.8 to get switch and case one can use
|
---|
493 | the Switch extension and say:
|
---|
494 |
|
---|
495 | use Switch;
|
---|
496 |
|
---|
497 | after which one has switch and case. It is not as fast as it could be
|
---|
498 | because it's not really part of the language (it's done using source
|
---|
499 | filters) but it is available, and it's very flexible.
|
---|
500 |
|
---|
501 | In addition to the above BLOCK construct, you could write
|
---|
502 |
|
---|
503 | SWITCH: {
|
---|
504 | $abc = 1, last SWITCH if /^abc/;
|
---|
505 | $def = 1, last SWITCH if /^def/;
|
---|
506 | $xyz = 1, last SWITCH if /^xyz/;
|
---|
507 | $nothing = 1;
|
---|
508 | }
|
---|
509 |
|
---|
510 | (That's actually not as strange as it looks once you realize that you can
|
---|
511 | use loop control "operators" within an expression. That's just the binary
|
---|
512 | comma operator in scalar context. See L<perlop/"Comma Operator">.)
|
---|
513 |
|
---|
514 | or
|
---|
515 |
|
---|
516 | SWITCH: {
|
---|
517 | /^abc/ && do { $abc = 1; last SWITCH; };
|
---|
518 | /^def/ && do { $def = 1; last SWITCH; };
|
---|
519 | /^xyz/ && do { $xyz = 1; last SWITCH; };
|
---|
520 | $nothing = 1;
|
---|
521 | }
|
---|
522 |
|
---|
523 | or formatted so it stands out more as a "proper" C<switch> statement:
|
---|
524 |
|
---|
525 | SWITCH: {
|
---|
526 | /^abc/ && do {
|
---|
527 | $abc = 1;
|
---|
528 | last SWITCH;
|
---|
529 | };
|
---|
530 |
|
---|
531 | /^def/ && do {
|
---|
532 | $def = 1;
|
---|
533 | last SWITCH;
|
---|
534 | };
|
---|
535 |
|
---|
536 | /^xyz/ && do {
|
---|
537 | $xyz = 1;
|
---|
538 | last SWITCH;
|
---|
539 | };
|
---|
540 | $nothing = 1;
|
---|
541 | }
|
---|
542 |
|
---|
543 | or
|
---|
544 |
|
---|
545 | SWITCH: {
|
---|
546 | /^abc/ and $abc = 1, last SWITCH;
|
---|
547 | /^def/ and $def = 1, last SWITCH;
|
---|
548 | /^xyz/ and $xyz = 1, last SWITCH;
|
---|
549 | $nothing = 1;
|
---|
550 | }
|
---|
551 |
|
---|
552 | or even, horrors,
|
---|
553 |
|
---|
554 | if (/^abc/)
|
---|
555 | { $abc = 1 }
|
---|
556 | elsif (/^def/)
|
---|
557 | { $def = 1 }
|
---|
558 | elsif (/^xyz/)
|
---|
559 | { $xyz = 1 }
|
---|
560 | else
|
---|
561 | { $nothing = 1 }
|
---|
562 |
|
---|
563 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
|
---|
564 | a temporary assignment to C<$_> for convenient matching:
|
---|
565 |
|
---|
566 | SWITCH: for ($where) {
|
---|
567 | /In Card Names/ && do { push @flags, '-e'; last; };
|
---|
568 | /Anywhere/ && do { push @flags, '-h'; last; };
|
---|
569 | /In Rulings/ && do { last; };
|
---|
570 | die "unknown value for form variable where: `$where'";
|
---|
571 | }
|
---|
572 |
|
---|
573 | Another interesting approach to a switch statement is arrange
|
---|
574 | for a C<do> block to return the proper value:
|
---|
575 |
|
---|
576 | $amode = do {
|
---|
577 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
|
---|
578 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
|
---|
579 | elsif ($flag & O_RDWR) {
|
---|
580 | if ($flag & O_CREAT) { "w+" }
|
---|
581 | else { ($flag & O_APPEND) ? "a+" : "r+" }
|
---|
582 | }
|
---|
583 | };
|
---|
584 |
|
---|
585 | Or
|
---|
586 |
|
---|
587 | print do {
|
---|
588 | ($flags & O_WRONLY) ? "write-only" :
|
---|
589 | ($flags & O_RDWR) ? "read-write" :
|
---|
590 | "read-only";
|
---|
591 | };
|
---|
592 |
|
---|
593 | Or if you are certain that all the C<&&> clauses are true, you can use
|
---|
594 | something like this, which "switches" on the value of the
|
---|
595 | C<HTTP_USER_AGENT> environment variable.
|
---|
596 |
|
---|
597 | #!/usr/bin/perl
|
---|
598 | # pick out jargon file page based on browser
|
---|
599 | $dir = 'http://www.wins.uva.nl/~mes/jargon';
|
---|
600 | for ($ENV{HTTP_USER_AGENT}) {
|
---|
601 | $page = /Mac/ && 'm/Macintrash.html'
|
---|
602 | || /Win(dows )?NT/ && 'e/evilandrude.html'
|
---|
603 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
|
---|
604 | || /Linux/ && 'l/Linux.html'
|
---|
605 | || /HP-UX/ && 'h/HP-SUX.html'
|
---|
606 | || /SunOS/ && 's/ScumOS.html'
|
---|
607 | || 'a/AppendixB.html';
|
---|
608 | }
|
---|
609 | print "Location: $dir/$page\015\012\015\012";
|
---|
610 |
|
---|
611 | That kind of switch statement only works when you know the C<&&> clauses
|
---|
612 | will be true. If you don't, the previous C<?:> example should be used.
|
---|
613 |
|
---|
614 | You might also consider writing a hash of subroutine references
|
---|
615 | instead of synthesizing a C<switch> statement.
|
---|
616 |
|
---|
617 | =head2 Goto
|
---|
618 | X<goto>
|
---|
619 |
|
---|
620 | Although not for the faint of heart, Perl does support a C<goto>
|
---|
621 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
|
---|
622 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for
|
---|
623 | a C<goto>; it's just the name of the loop.
|
---|
624 |
|
---|
625 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
|
---|
626 | execution there. It may not be used to go into any construct that
|
---|
627 | requires initialization, such as a subroutine or a C<foreach> loop. It
|
---|
628 | also can't be used to go into a construct that is optimized away. It
|
---|
629 | can be used to go almost anywhere else within the dynamic scope,
|
---|
630 | including out of subroutines, but it's usually better to use some other
|
---|
631 | construct such as C<last> or C<die>. The author of Perl has never felt the
|
---|
632 | need to use this form of C<goto> (in Perl, that is--C is another matter).
|
---|
633 |
|
---|
634 | The C<goto>-EXPR form expects a label name, whose scope will be resolved
|
---|
635 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
|
---|
636 | necessarily recommended if you're optimizing for maintainability:
|
---|
637 |
|
---|
638 | goto(("FOO", "BAR", "GLARCH")[$i]);
|
---|
639 |
|
---|
640 | The C<goto>-&NAME form is highly magical, and substitutes a call to the
|
---|
641 | named subroutine for the currently running subroutine. This is used by
|
---|
642 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then
|
---|
643 | pretend that the other subroutine had been called in the first place
|
---|
644 | (except that any modifications to C<@_> in the current subroutine are
|
---|
645 | propagated to the other subroutine.) After the C<goto>, not even C<caller()>
|
---|
646 | will be able to tell that this routine was called first.
|
---|
647 |
|
---|
648 | In almost all cases like this, it's usually a far, far better idea to use the
|
---|
649 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
|
---|
650 | resorting to a C<goto>. For certain applications, the catch and throw pair of
|
---|
651 | C<eval{}> and die() for exception processing can also be a prudent approach.
|
---|
652 |
|
---|
653 | =head2 PODs: Embedded Documentation
|
---|
654 | X<POD> X<documentation>
|
---|
655 |
|
---|
656 | Perl has a mechanism for intermixing documentation with source code.
|
---|
657 | While it's expecting the beginning of a new statement, if the compiler
|
---|
658 | encounters a line that begins with an equal sign and a word, like this
|
---|
659 |
|
---|
660 | =head1 Here There Be Pods!
|
---|
661 |
|
---|
662 | Then that text and all remaining text up through and including a line
|
---|
663 | beginning with C<=cut> will be ignored. The format of the intervening
|
---|
664 | text is described in L<perlpod>.
|
---|
665 |
|
---|
666 | This allows you to intermix your source code
|
---|
667 | and your documentation text freely, as in
|
---|
668 |
|
---|
669 | =item snazzle($)
|
---|
670 |
|
---|
671 | The snazzle() function will behave in the most spectacular
|
---|
672 | form that you can possibly imagine, not even excepting
|
---|
673 | cybernetic pyrotechnics.
|
---|
674 |
|
---|
675 | =cut back to the compiler, nuff of this pod stuff!
|
---|
676 |
|
---|
677 | sub snazzle($) {
|
---|
678 | my $thingie = shift;
|
---|
679 | .........
|
---|
680 | }
|
---|
681 |
|
---|
682 | Note that pod translators should look at only paragraphs beginning
|
---|
683 | with a pod directive (it makes parsing easier), whereas the compiler
|
---|
684 | actually knows to look for pod escapes even in the middle of a
|
---|
685 | paragraph. This means that the following secret stuff will be
|
---|
686 | ignored by both the compiler and the translators.
|
---|
687 |
|
---|
688 | $a=3;
|
---|
689 | =secret stuff
|
---|
690 | warn "Neither POD nor CODE!?"
|
---|
691 | =cut back
|
---|
692 | print "got $a\n";
|
---|
693 |
|
---|
694 | You probably shouldn't rely upon the C<warn()> being podded out forever.
|
---|
695 | Not all pod translators are well-behaved in this regard, and perhaps
|
---|
696 | the compiler will become pickier.
|
---|
697 |
|
---|
698 | One may also use pod directives to quickly comment out a section
|
---|
699 | of code.
|
---|
700 |
|
---|
701 | =head2 Plain Old Comments (Not!)
|
---|
702 | X<comment> X<line> X<#> X<preprocessor> X<eval>
|
---|
703 |
|
---|
704 | Perl can process line directives, much like the C preprocessor. Using
|
---|
705 | this, one can control Perl's idea of filenames and line numbers in
|
---|
706 | error or warning messages (especially for strings that are processed
|
---|
707 | with C<eval()>). The syntax for this mechanism is the same as for most
|
---|
708 | C preprocessors: it matches the regular expression
|
---|
709 |
|
---|
710 | # example: '# line 42 "new_filename.plx"'
|
---|
711 | /^\# \s*
|
---|
712 | line \s+ (\d+) \s*
|
---|
713 | (?:\s("?)([^"]+)\2)? \s*
|
---|
714 | $/x
|
---|
715 |
|
---|
716 | with C<$1> being the line number for the next line, and C<$3> being
|
---|
717 | the optional filename (specified with or without quotes).
|
---|
718 |
|
---|
719 | There is a fairly obvious gotcha included with the line directive:
|
---|
720 | Debuggers and profilers will only show the last source line to appear
|
---|
721 | at a particular line number in a given file. Care should be taken not
|
---|
722 | to cause line number collisions in code you'd like to debug later.
|
---|
723 |
|
---|
724 | Here are some examples that you should be able to type into your command
|
---|
725 | shell:
|
---|
726 |
|
---|
727 | % perl
|
---|
728 | # line 200 "bzzzt"
|
---|
729 | # the `#' on the previous line must be the first char on line
|
---|
730 | die 'foo';
|
---|
731 | __END__
|
---|
732 | foo at bzzzt line 201.
|
---|
733 |
|
---|
734 | % perl
|
---|
735 | # line 200 "bzzzt"
|
---|
736 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
|
---|
737 | __END__
|
---|
738 | foo at - line 2001.
|
---|
739 |
|
---|
740 | % perl
|
---|
741 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
|
---|
742 | __END__
|
---|
743 | foo at foo bar line 200.
|
---|
744 |
|
---|
745 | % perl
|
---|
746 | # line 345 "goop"
|
---|
747 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
|
---|
748 | print $@;
|
---|
749 | __END__
|
---|
750 | foo at goop line 345.
|
---|
751 |
|
---|
752 | =cut
|
---|