### filehandle = openfile( filename )
sub openfile {
  my $filename = shift;
  
  if ( $filename =~ m{\.bz2\z} ) {
    open my $fh, '-|', "bzcat $filename" or die "bzcat $filename: $!\n";
  }
  elsif ( $filename =~ m{\.gz\z} ) {
    open my $fh, '-|', "zcat $filename" or die "zcat $filename: $!\n";
  }
  else {
    open my $fh, '<', $filename or die "open $filename: $!\n";
  }

  return $fh;
}