For details on the behaviour of BEGIN blocks, see the perlmod man page under the heading "Package Constructors and Destructors". For an explanation of auto-vivification, see perlref, perlfaq4, perlfunc and the node Autovivification by merlyn.

Update: Erk, weirder stuff appearing - It almost looks like something weird is happening with closures ...

I don't understand. Why would $guid{218827} be auto-vivified? Note that $guid{218827} is referenced from a subroutine, which isn't executed until after the BEGIN block.

I think there's a bug in Perl. Watch what happens if you dump the content of %guid in the BEGIN block:

#!/usr/bin/perl
     
use warnings;
         
            
my %guid=(218827=>5,109940=>2);
          
BEGIN 
{
    %handle=
    (
        cmd => sub
        {
            my $ref = sub{$guid{218827}++;warn "sub: $guid{218827}";};
            $ref->();
        },
    );
    while (my ($key, $value) = each %guid) {
        print defined $key ? $key : "UNDEF";
        print ":";
        print defined $value ? $value : "UNDEF";
        print "\n";
    }
}        
            

$handle{cmd}->();
__END__
sub: 6 at buu line 14.
[download]

Not only does it show that the BEGIN block doesn't autovivify the value, the error disappears as well!

In fact, just about any reference to %guid (like keys %guid;, %guid;, or $guid {1};) after the assignment to %handle makes the error go away.

This ought to be perlbugged.

Abigail

It appears to be fixed in bleadperl. Nevertheless, a bug report with a short test case (or patch to the t/op/closure.t) would be good.

my $x=5;
BEGIN {$y=sub{my $r=sub{$x++;warn "sub: $x"};$r->()}}
$y->();
#sub: 1 at temp.pl line 2.
#deref sub that derefs sub, my before BEGIN, using scalars => autoviv

BEGIN {$y=sub{my $r=sub{$x++;warn "sub: $x"};$r->()}}
my $x=5;
$y->();
#sub: 1 at temp.pl line 1.
#deref sub that derefs sub, BEGIN before my, using scalars => autoviv


my %x=(1=>5);
BEGIN {$y=sub{my $r=sub{$x{1}++;warn "sub: $x{1}"};$r->()}}
$y->();
#Modification of a read-only value attempted at temp.pl line 2.
#deref sub that derefs sub, my before BEGIN, using hash => ERROR

BEGIN {$y=sub{my $r=sub{$x{1}++;warn "sub: $x{1}"};$r->()}}
my %x=(1=>5);
$y->();
#sub: 1 at temp.pl line 1.
#deref sub that derefs sub, BEGIN before my, using hash => autoviv


my %x=(1=>5);
BEGIN {$y=sub{$x{1}++;warn "sub: $x{1}";}}
$y->();
#sub: 6 at temp.pl line 2.
#deref sub, my before BEGIN, using hash => uses main's my variable

BEGIN {$y=sub{$x{1}++;warn "sub: $x{1}";}}
my %x=(1=>5);
$y->();
#sub: 1 at temp.pl line 1.
#deref sub, BEGIN before my, using hash => autoviv


my $x=5;
BEGIN {$y=sub{$x++;warn "sub: $x"}}
$y->();
#sub: 6 at temp.pl line 2.
#deref sub, my before BEGIN, using scalar => uses main's my variable

BEGIN {$y=sub{$x++;warn "sub: $x"}}
my $x=5;
$y->();
#sub: 1 at temp.pl line 1.
#deref sub, BEGIN before my, using scalar => autoviv
[download]

I don't know how to submit a bug report so please feel free to submit one using my test cases.

AS 5.8.2 808 / win2k

Then this is a wonderful time to learn how to submit a bug report! It takes just a few minutes; start by reading perldoc perlbug.