fork() is problematic with Tk applications, as the child process can't munge with the parent's window hierarchy. You could fork(), and then have the child feed the data over a pipe or socketpair...seems complicated though.

There is Tk::fileevent, which gets you most of the way there, but doesn't have a built-in mechanism to re-seek for the 'tail -f' functionality you need. Basically the fileevent fires on eof, so you would have to turn the fileevent on and off. Ick.

My last idea was to use Tk::After's repeat() functionality. Seems to work for me. This short example creates a listbox, and does something of a "tail -f /tmp/testfile" on it while still allowing other events to fire. (You can type in the entry box to verify this)

Here you go:

#!/usr/bin/perl
use IO::File;
use IO::Select;
use Tk;
my $FILE=IO::File->new("/tmp/testfile") or die;
my $SEL=IO::Select->new();
$SEL->add($FILE);
my $mw=Tk::MainWindow->new();
my $list=$mw->Listbox->pack();
my $entry=$mw->Entry->pack();
$list->repeat(500, sub {
    if ($SEL->can_read(0)) {
         while(<$FILE>) {
             chomp;
             $list->insert(end,$_);
             $list->see(($list->size)-1);
         }
         seek(FILE,0,1);
    }
});
MainLoop;
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Update: Depending on how fast the text is streaming, you may want to call Tk::DoOneEvent(0) in the while(<$FILE>) loop.