I tried your benchmark and I also see map is faster. When I changed subs to return concatenated results, for loop becomes faster. I wonder why?

#!/usr/bin/perl
use strict; use warnings;
use Benchmark qw/cmpthese/;

my %h = map {$_ => $_} 1 .. 10000;

sub test1{
    my $buff='';
    foreach my $key (sort keys %h) {
        $buff.="$key: $h{$key}\n";
    }
    return $buff;
}
sub test2{
    return join('', map "$_: $h{$_}\n", sort keys %h);
}

print test1() eq test2() ? "same\n" : "not same\n";

my %tests = (
    '01_for' => \&test1,
    '02_map' => \&test2,
);
cmpthese(
    -10, #for 10 cpu secs
    \%tests
);

__DATA__
         Rate 02_map 01_for
02_map 33.9/s     --   -16%
01_for 40.2/s    19%     --
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