There are a few variations on this, but basically you do a piped-open of the tail command, then run the filehandle thru Tk::fileeevnt. But here is an even easier way.

#!/usr/bin/perl
use strict;
use Tk;
use IO::Handle;

my $H=IO::Handle->new;
open($H,"tail -f -n 50 z.txt |") or die $!;

my $main = MainWindow->new;
my $t=$main->Text(-wrap=>'none')->pack(-expand=>1);
$main->fileevent(\*$H,'readable',[\&fill,$t]);

MainLoop;

sub fill {
  my ($w)=@_;
  my $text;
my $text =<$H>;
$w->insert('end',$text);
$w->yview('end');
}
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