use IO::Handle;
use Fcntl;
use POSIX qw(exit);

$SIG{PIPE} = 'IGNORE';

my $handle = IO::Handle-> new;
$handle-> autoflush(1);
my $pid = open( $handle, '-|');
if ( $pid) {
     # parent

     # make it non-blocking
     my $fl = fcntl( $handle, F_GETFL, 0);
     die "$!" unless defined $fl;
     fcntl( $handle, F_SETFL, $fl|O_NONBLOCK) or die "$!";

     # attach to your gui system so your callbacks are
     # pinged whenever something appears on $handle
     MyFictitiousGUI::FileListener->attach( $handle, 
        on_read => sub { 
            message( <$handle>);
        },
        on_close => sub {
            message("it's over!")
        }
     );
} else {
     # child
     $|++;
     while ( do_computation) {
         print "$percents done\n";
     }
     POSIX::_exit(0);
}