use strict;
use warnings;

my %input = (
	r => [1..10],
	s => [1..10],
	t => [5..10],
	u => [11..20],
);

for my $outer (sort keys %input) {
	next if ! $input{$outer};
	my %seen = map {$_ => 1} @{$input{$outer}};
	for my $inner (grep {$_ ne $outer} sort keys %input) {
		if (! grep {!$seen{$_}} @{$input{$inner}}) {
			delete $input{$inner};
		}
	}
}

print "$_\n" for sort keys %input;