if ($fh == $ircsock) {
  ...
  if (sysread($ircsock, $ircdata, 512)) {
     ...
  } else {
    $sel->remove($ircsock);
    close($ircsock);
    ...reopen $ircsock...
    ...add $ircsock to $sel...
  }

##</code><code>##

my $clientdata = "";

while (1) {
  #-- Add the new data at the end of the existing one
  sysread $fh, $clientdata, 512, length $ircdata;

  #-- See if we have a full line we can process
  while ($fh =~ s/^(.*?)\r?\n//) {
  my $line = $1;

  ... do your per-line regex stuff here...
  }
}