#!/usr/bin/perl -l

use strict;
use warnings;
no warnings qw/uninitialized/;


for (
  [ "1,7", "2,6", "2,7", "5,4", "6,7" ],
  [ "1,7", "2,6", "2,7", "3,4", "6,7" ],
  [ "1,7", "2,6", "2,7", "3,4", "9,8" ],
  [ "a,b", "c,b", "d,f", "e,b", "f,g" ],
) {

  my @pairs = @$_;

  my $count = 0;
  my %sets;
  my %sets2;

  for my $p (@pairs) {
    my ($l, $r) = split /\s*,\s*/, $p;

#    print "P = $p, L = $l, R = $r, \$sets{\$l} = $sets{$l}, \$sets{\$r} = $sets{$r}";
    if ($sets{$l} and $sets{$r} and $sets{$l} != $sets{$r}) {
      # $sets{$l} and $sets{$r} can now be linked
      
      my $old_set = $sets{$r};
      for my $s (keys %sets) {
        if ($sets{$s} == $old_set) {
          if (exists $sets2{ $sets{$s} }) {
            push @{ $sets2{ $sets{$l} } }, @{ $sets2{ $sets{$s} } };
            delete $sets2{ $old_set };
          }

          $sets{$s} = $sets{$l};
        }
      }
    }
    elsif ($sets{$l}) {
      $sets{$r} = $sets{$l};
    }
    elsif ($sets{$r}) {
      $sets{$l} = $sets{$r};
    }
    else {
      $count++;
      $sets{$l} = $sets{$r} = $count;
    }
    push @{ $sets2{ $sets{$l} } }, $p;
  }

  print "SETS FOR: " . join(", ", @pairs);
  my $i = 0;
  for my $key (keys %sets2) {
    print "\tSET $i: " . join(", ", @{ $sets2{ $key} });
    $i++;
  }
  print "";
}

__END__
Output:

SETS FOR: 1,7, 2,6, 2,7, 5,4, 6,7
        SET 0: 5,4
        SET 1: 2,6, 1,7, 2,7, 6,7

SETS FOR: 1,7, 2,6, 2,7, 3,4, 6,7
        SET 0: 3,4
        SET 1: 2,6, 1,7, 2,7, 6,7

SETS FOR: 1,7, 2,6, 2,7, 3,4, 9,8
        SET 0: 9,8
        SET 1: 3,4
        SET 2: 2,6, 1,7, 2,7

SETS FOR: a,b, c,b, d,f, e,b, f,g
        SET 0: a,b, c,b, e,b
        SET 1: d,f, f,g