use IO::Handle;
my $fh = *STDIN;
$fh->blocking(0);
while (1) {  # this block would go in your callback, not in a loop like this:
  $fh->sysread( my $data, 255 ); 
  print "$data";   # split $data into lines on CR|LF put the first line on the end of the last line of the previous block (use an array perhaps)
}