#!/usr/bin/perl

# https://perlmonks.org/?node_id=1222680

use strict;
use warnings;

while( <DATA> )
  {
  my ($cluster, $member) = split;
  print $cluster eq $member ? "\n" x ($. > 1) : ', ', $member;
  }
print "\n";

__DATA__
Osat_a    Osat_a # just one cluster member
Atha_b    Atha_b # >1 cluster member, this & next line = 2 members
Atha_b    Mtru_c
Fves_d    Fves_d # this & next 2 lines = 3 cluster members
Fves_d    Osat_e
Fves_d    Atha_f
Atha_g    Atha_g # just 1 cluster member
Osat_h    Osat_h
Osat_h    Atha_i
Mtru_j    Mtru_j # just 1 cluster member