This ends up with some weird behaviour. I'm not exactly sure whats happening, but i dont think the param \%hash is being passed to the sorter in a manner that we (I) think it is.

Try this:

my %hash = (
   1 => 'A',
   2 => 'B',
   3 => 'C',
);

foreach my $key (sort mysort(\%hash), keys %hash) {
   print "$key\n";
}


sub mysort {
   print "mysort: \$_[0] = $_[0] ::: a = $a ::: b = $b\n";
   return $_[0]->{$a} cmp $_[0]->{$b};
}
[download]

...lots of warnings and unexpected things.