use strict;
use warnings;

my @array = map {[split]} ('11 12', '11 13', '9 8 7', '3 4', '11 4');

ORIG: for (my $i=0; $i<=$#array; $i++) {
    for my $j ($i+1..$#array) {
        my %seen;
        my @unique = grep {! $seen{$_}++} @{$array[$i]}, @{$array[$j]};
        if (@unique < @{$array[$i]} + @{$array[$j]}) {
            $array[$i] = \@unique;
            splice @array, $j, 1;
            redo ORIG;
        }
    }
    $array[$i] = join ' ', @{$array[$i]};
}

print "$_\n" for (@array);