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

$sorter = \&my_sorter;
foreach my $key (sort { &$sorter(\%hash) } keys %hash ) {
...
    my ($hashref) = @_;
    return $hashref->{$a} cmp $hashref->{$b};
}

$sorter = \&my_sorter;
foreach my $key (
...
sub my_sorter {
    return $a->[1] cmp $b->[1];
}