my %hGroups = (
          'Group1' => {
                        'ATRG7' => 1,
                        'ATG2' => 1,
                        'ATG4' => 1,
                        'ATG1' => 1
                      },
          'Group3' => {
                        'FYCO1' => 1,
                        'LSM2' => 1
                      },
          'Group2' => {
                        'ATG9' => 1,
                        'ATG1' => 1
                      }
        );

##</code><code>##

my %hGroups = map { my $sGenes = $hash{$_};
                    my $hGroupMembers
                      = { map { $_ => 1 } split(',', $sGenes) };
                    $_ => $hGroupMembers;
                  } keys %hash;

##</code><code>##

my %hNewGroups;

oldgroup:
foreach my $sGroup (keys %hGroups) {
  my $hGroupMembers = $hGroups{$sGroup};

  # check each new group for genes in common with
  # current old group ($sGroup)

  foreach my $sNewGroup (keys %hNewGroups) {

    # check genes in the old group to see if any are
    # in the new group ($sNewGroup).
    # Note: use exists to prevent auto-vivification
    # (automatic adding) of $sGene to the members hash

    my $hNewGroupMembers = $hNewGroups{$sNewGroup};
    foreach my $sGene (keys %$hGroupMembers) {
      if (exists($hNewGroupMembers->{$sGene})) {
        $hNewGroups{$sNewGroup} = { %$hNewGroupMembers
                                    , %$hGroupMembers };
        next oldgroup;
      }
    }
  }

  # create a new group, since no gene is in common with
  # other groups found so far

  $hNewGroups{$sGroup} = $hGroupMembers;
}

##</code><code>##

%hNewGroups = (
          'Group1' => {
                        'ATG9' => 1,
                        'ATG2' => 1,
                        'ATRG7' => 1,
                        'ATG4' => 1,
                        'ATG1' => 1
                      },
          'Group3' => {
                        'FYCO1' => 1,
                        'LSM2' => 1
                      }
        );

##</code><code>##

while (my ($sNewGroup,$hMembers) = each(%hNewGroups)) {
  print "$sNewGroup: " . join(',', keys %$hMembers) . "\n";
}

##</code><code>##

Group1: ATG9,ATG2,ATRG7,ATG4,ATG1
Group3: FYCO1,LSM2