in reply to What is the scope of $_?

There is more than one $_. All your examples use the global $_. This is global in scope (as are all the "punctuation variables", except @_ which is just weird).

However, certain constructs implicitly localize variables. for loops do this.

Also (from Perl 5.10) you can create a new variable called $_ using my $_ or given (though the behaviour of given will change in 5.18 - it will not implicitly create a lexical $_). These have the same scope as any other lexical variables - the remainder of the block they're defined in, where the file itself counts as a block.

If you want to access your caller's lexical $_, then the official way is to use a sub prototype. I've also written lexical::underscore which allows you to delve multiple levels down the call stack.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

Replies are listed 'Best First'.
Re^2: What is the scope of $_?
by kennethk (Abbot) on Nov 26, 2012 at 18:40 UTC
    <tangent> How is @_ not global in scope? I mean, it gets implicitly localized on any subroutine call, but it's present by default and package independent: 
    use strict;
use warnings;

@_ = @ARGV;

package Foo;

print for @_;
@_ = qw(1 2 3 4 5);
{
    local @_ = reverse @_;
    print for @_;
}

package main;

print for @_;

    [download]
    </tangent>

    #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

[reply]
[d/l]
[select]

      You are correct. I was under the impression that a reference to @_ behaved differently than a reference to another localized array, once the localization went out of scope.

      But the following example appears to show they behave the same...

      use strict;
use Data::Dumper;

our @X;
my ($underscore, $scissors);

@_ = (1..3);
@X = (1..3);

sub foo {
    local @X = (4..6);
    $underscore = \@_;
    $scissors   = \@X;
}

foo(4..6);
print Dumper($underscore, $scissors);

      [download]

      So (like $_) @_ is just a global array that gets localized by certain control structures. (In particular, sub.)

      perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
[reply]
[d/l]
[select]
Re^2: What is the scope of $_?
by Lotus1 (Vicar) on Nov 26, 2012 at 19:46 UTC

    If all the OP's examples use the global $_ then printing it at the end would show the last value assigned. What actually happens is a warning for 'Use of uninitialized value $_ ...' The subroutine uses whatever $_ that is in scope when it is called. 

    use warnings;
use strict;

sub abc()
{
  $_ = 'x';
}

foreach(1..2)
{
    my $loopvar = 'in the loop';
  print "Before:$_\n";
  abc();
  print "After:$_\n";
}

print "\$_ = $_\n";  # warning for uninitialized value $_ (global scop
+e)

abc();
print "\$_ = $_\n";  # prints $_ = x

    [download]
[reply]
[d/l]
[select]

      As I said, all the OP's examples use the global $_ but some Perl control structures localize that variable.

      The same effect is illustrated here using global $::x...

      use warnings;
use strict;

sub abc()
{
    $::x = 'x';
}

foreach $::x (1..2)
{
    print "Before:$::x\n";
    abc();
    print "After:$::x\n";
}

print "\$::x = $::x\n";  # warning for uninitialized value $x (global 
+scope)

abc();
print "\$::x = $::x\n";  # prints $x = x

      [download]
      perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
[reply]
[d/l]
[select]

        So when the foreach loop finishes it restores the undef value to $_. Thanks.

[reply]

In Section Seekers of Perl Wisdom