sub grepit {
  my ( $fh, $keep ) = @_;
  my $new_fh;
  return $new_fh if my $pid = open $new_fh, '-|';
  die "Fork failed: $!\n" unless defined $pid;
  my $re = ref $keep ? $keep : qr/\Q$keep/;
  /$re/ && print STDOUT while <$fh>;
  exit 0;
}

sub cols {
  my ( $fh, $sep, @cols ) = @_;
  my $new_fh;
  return $new_fh if my $pid = open $new_fh, '-|';
  die "Fork failed: $!\n" unless defined $pid;
  print STDOUT join( $sep, ( split $sep )[ @cols ] ), "\n"
    while <$fh>;
  exit 0;
}