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;
}