First, the following allow you to switch the procedure being called:

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

Update:

Because %hash is not necessarily scoped in my_sorter

Option 1

$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};
}
[download]

Option 2

$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];
}
[download]

Option 3

Use local %hash = %the_hash_to_process; before the foreach, and our %hash inside my_sorter.

There are more ways.

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