That is an interesting case BEGIN { my $x; sub foo { ... $x ... } } Perl could decide to implement that as a closure (and probably does because "BEGIN" blocks are implemented as subroutines). But lets find out: 

#!/usr/bin/perl -w
use strict;

use Devel::Peek qw(Dump);

BEGIN {
    my $x;
    sub begin {
        ++$x;
    }
}

sub justmy {
    my $x;
    ++$x;
}

sub ifmy {
    my $x if 0;
    ++$x;
}

{
    my $x;
    sub static {
        ++$x;
    }
}

sub nest {
    my $x;
    sub inner {
        ++$x
    }
}

sub gen {
    my $x;
    return sub { ++$x };
}

*insert= gen();

Dump $_
    for( \&begin, \&justmy, \&ifmy, \&static, \&inner, gen(), \&insert
+ );

[download]
The "cleaned up" output is: 
Variable "$x" will not stay shared at closure.pl line 27.

begin:
    SV = RV(0x1a83a20) at 0x1a65068
      SV = PVCV(0x1a8340c) at 0x1a62144
        GVGV::GV = 0x1a7b6c0        "main" :: "begin"
        PADLIST = 0x1a7b690
          1. 0x1a621b0 (FAKE "$x" 0-57)
        OUTSIDE = 0x1a620fc (UNIQUE)
justmy:
    SV = RV(0x1a83a2c) at 0x1a65098
      SV = PVCV(0x1a8345c) at 0x1a620f0
        GVGV::GV = 0x1a7b72c        "main" :: "justmy"
        PADLIST = 0x1a7b708
          1. 0x1a7b714 ("$x" 59-60)
        OUTSIDE = 0x1a6f124 (MAIN)
ifmy:
    SV = RV(0x1a83a30) at 0x1a650b0
      SV = PVCV(0x1a7adbc) at 0x1a7b750
        GVGV::GV = 0x1a7b798        "main" :: "ifmy"
        PADLIST = 0x1a7b774
          1. 0x1a7b780 ("$x" 61-62)
        OUTSIDE = 0x1a6f124 (MAIN)
static:
    SV = RV(0x1a839dc) at 0x1a650e0
      SV = PVCV(0x1a7ada4) at 0x1a7b76c
        GVGV::GV = 0x1a7b7a8    "main" :: "static"
        PADLIST = 0x1a7b790
          1. 0x1a620f0 (FAKE "$x" 0-64)
        OUTSIDE = 0x1a6f124 (MAIN)
inner:
    SV = RV(0x1a83a34) at 0x1a650c8
      SV = PVCV(0x1a7ae5c) at 0x1a7b81c
        GVGV::GV = 0x1a7b858        "main" :: "inner"
        PADLIST = 0x1a7b840
          1. 0x1a7b7ec (FAKE "$x" 0-64)
        OUTSIDE = 0x1a7b7bc (nest)
gen():
    SV = RV(0x1a83a38) at 0x1a65158
      SV = PVCV(0x1a7af9c) at 0x1a650e0
        FLAGS = (ANON,CLONED)
        GVGV::GV = 0x1a7b924        "main" :: "__ANON__"
        PADLIST = 0x1a65128
          1. 0x1a65080 (FAKE "$x" 0-67)
        OUTSIDE = 0x1a7b894 (gen)
        SV = PVCV(0x1a7aeac) at 0x1a7b894
          GVGV::GV = 0x1a7b93c      "main" :: "gen"
          PADLIST = 0x1a7b8b8
            1. 0x1a65170 ("$x" 66-68)
            2. 0x1a7b8e8 ("&" 1--1)
          OUTSIDE = 0x1a6f124 (MAIN)
insert:
    SV = RV(0x1a83a3c) at 0x1a65188
      SV = PVCV(0x1a7af4c) at 0x1a6f01c
        FLAGS = (ANON,CLONED)
        GVGV::GV = 0x1a7b924        "main" :: "__ANON__"
        FLAGS = 0x6
        PADLIST = 0x1a6f0f4
          1. 0x1a7b8c4 (FAKE "$x" 0-67)
        OUTSIDE = 0x1a7b894 (gen)
        SV = PVCV(0x1a7aeac) at 0x1a7b894
          GVGV::GV = 0x1a7b93c      "main" :: "gen"
          PADLIST = 0x1a7b8b8
            1. 0x1a65170 ("$x" 66-68)
            2. 0x1a7b8e8 ("&" 1--1)
          OUTSIDE = 0x1a6f124 (MAIN)

[download]
or just consider 
begin:    1. 0x1a621b0 (FAKE "$x" 0-57)
justmy:   1. 0x1a7b714 ("$x" 59-60)
ifmy:     1. 0x1a7b780 ("$x" 61-62)
static:   1. 0x1a620f0 (FAKE "$x" 0-64)
inner:    1. 0x1a7b7ec (FAKE "$x" 0-64)
gen():    1. 0x1a65080 (FAKE "$x" 0-67)
insert:   1. 0x1a7b8c4 (FAKE "$x" 0-67)

[download]
which seems to indicates that "justmy" and "ifmy" are not implemented as closures but all of the rest are implemented as closures. So I'll certainly be more lenient in what I let other people call closures. (:

But it also indicates that the padlist is carried around for ordinary subroutines, which makes that aspect of the implementation less important to me.

I think that the important thing about closures is being able to call the same code but have it use different variables (without passing them in as arguments). Above, only the anonymous subroutine and "insert" meet that criterion. So those are what I'll call closures. The other 3 cases I'll call "static variables that Perl implements via closures" if pushed. :)

        - tye (but my friends call me "Tye")

In reply to (tye)Re2: A Real Closure by tye
in thread Unusual Closure Behaviour by tachyon

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