Hello plagent,

In the spirit of TMTOWTDI: You can also use one of the Set modules, such as Set::Tiny:

#! perl
use strict;
use warnings;
use Set::Tiny qw( set );

my   @groups;
push @groups, set(split) while <DATA>;

OUTER: while (1)
{
    for my $i (0 .. $#groups - 1)
    {
        for my $j ($i + 1 .. $#groups)
        {
            unless ($groups[$i]->is_disjoint($groups[$j]))
            {
                $groups[$i] = $groups[$i]->union($groups[$j]);
                splice @groups, $j, 1;
                next OUTER;
            }
        }
    }

    last;
}

print $_->as_string, "\n" for @groups;

__DATA__
A   C
C   E
D   C
L   G
J   K
O   P
I   M
L   K
[download]

Output:

17:47 >perl 1468_SoPW.pl
(A C D E)
(G J K L)
(O P)
(I M)

17:47 >
[download]

Hope that helps,