Tk has a function built in called fileevent that is made for stuff like this.

Here's an example pretty much right out of "Learning Perl/Tk" from O'Reilly.

use Tk;
my $mw = MainWindow->new();
my $text = $mw->Scrolled("Text",
            -width => 80,
            -height => 25)->pack(-expand => 1);
$mw->fileevent(STDIN, 'readable', [\&insert_text]);
MainLoop;

sub insert_text
{
    my $curline=<STDIN>;
    $text->insert('end', $curline);
    
}
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That seems to work very nicely with a tail command piping output to the script. The other option of course, would be to open tail as a filehandle, like:

open(FH,"tail -f -n 1 logifle") || die "Could not tail: $!";
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Then use <FH> instead of <STDIN> in the above.

P.S.
My wife now hates you, I've often wondered how to do similar things it Tk, but never had a driving need. Now that I looked up an answer to your question, I might just start messing around with Tk again (I can see lots of possibilites!)