use FileHandle;
...
# Filehandle anlegen
my $fh = new FileHandle;

# Pipe-open
open ($fh, "das_pruefende_programm |") or die "ups... $!";

# Fileevent mit Callback verbinden
$MainWindow->fileevent ($fh, 'readable', \&funktion_die_was_macht);

sub funktion_die_was_macht {
    print <$fh>;   # Oder was anderes, z.B. den Text in ein Widget einfuegen
}

##### worker.pl #####
#!/usr/bin/perl
use strict;
use warnings;

$|++;   # ungepuffert schreiben

my $i=0;
while (1) {
        print ++$i, "\n";
        sleep 1;
}



##### caller.pl #####
#!/usr/bin/perl
use strict;
use warnings;
use Tk;
use FileHandle;

my $Mw = MainWindow->new (-title => 'Pipetest');
my $LogWin = $Mw->Text (-width => 80, -height => 20)->pack;
$Mw->Button (-text => 'Exit', -command => sub { exit })->pack;

# Filehandle anlegen
my $fh = new FileHandle;

# Pipe-open
open ($fh, "./worker.pl |") or die "ups... $!";

# Fileevent mit Callback verbinden
$Mw->fileevent ($fh, 'readable', \&funktion_die_was_macht);

$Mw->MainLoop;

###############

sub funktion_die_was_macht {
    $LogWin->insert ('end', "Lese " . <$fh>);
}

#####