$sorter = \&my_sorter;
foreach my $key (sort { &$sorter } keys %hash ) {
   # do something useful
}

##</code><code>##

$sorter = \&my_sorter;
foreach my $key (sort { &$sorter(\%hash) } keys %hash ) {
   # do something useful
}

sub my_sorter {
    my ($hashref) = @_;
    return $hashref->{$a} cmp $hashref->{$b};
}

##</code><code>##

$sorter = \&my_sorter;
foreach my $key (
   map { $_->[0] }
   sort { &$sorter }
   map { [ $_, $hash{$_} ] }
   keys %hash
) {
   # do something useful
}

sub my_sorter {
    return $a->[1] cmp $b->[1];
}