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...
  }
}