add my (%h_delete, %h_list, %h_undef); at the appropriate place near the top of the script,

The important lesson to be learned is that one should never blindly apply my to variables. Always know what you are doing, and always consider the consequences.

Abigail

Well, you got me that time. Of course I know what 'my' does. I didn't plan for this format to create a completely different lexical scope:

timethese 10000, {
    this => ' ...... ',
    that => ' ...... '
};
[download]

When benchmarking I'm accustomed to using the sub format, like this:

timethese 10000, {
    this => sub { ..... },
    that => sub { ..... }
}
[download]

...which follows a more natural lexical scoping, where the subs are still executed within the same scope as the lexical block in which they're called.

For the record, the following code now matches the OP's observed behavior:

use strict;
use warnings;
use Benchmark;

my (%h_delete, %h_list, %h_undef);
%h_delete = %h_list = %h_undef = (1 .. 50000);

delete @h_delete{keys %h_delete};
%h_list = ();
undef %h_undef;

timethese 10000, {
    delete  => sub{ 1 for keys %h_delete},
    list    => sub{ 1 for keys %h_list},
    undef   => sub{ 1 for keys %h_undef},
    };
[download]




Dave

I tend to not use the sub variant, but the string variant. Using a sub means entering and leaving a scope, and that takes time. If that time is significant compared to what you are benchmarking, your results are far less useful than they could be. Watch:

#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw /cmpthese/;

our ($x, $y);

cmpthese -1  => {
    sub      => sub {$::x ++},
    string   =>     '$::y ++',
}

__END__
             Rate    sub string
sub     9175039/s     --   -29%
string 12862205/s    40%     --
[download]

Abigail