$pid = open (INPUT, "mycommand $somefile |") or die ...;
while (<INPUT>) {
  if ( ... trigger to stop ... ) {
    kill 9, $pid;
    last;
  }
}
close INPUT;

##</code><code>##

$pid = open (INPUT, "gzip -dc $somegzfile | mycommand |") or die ...;
while (<INPUT>) {
  if ( ... trigger to stop ... ) {
    kill 9, $pid;
    last;
  }
}
close INPUT;