I'd use Tk::after - it has millisecond granularity.

While I appreciate that the code you posted may well be a highly simplified version of your real application, you may not need Tk::fileevent at all. See my example code where it wasn't needed.

#!/usr/bin/env perl

use strict;
use warnings;

use Tk;

my $mw = MainWindow->new();

my $action_F = $mw->Frame()->pack(-side => 'bottom');
$action_F->Button(-text => 'Exit', -command => sub { exit })->pack;

my $text_F = $mw->Frame()->pack(-fill => 'both', -expand => 1);
my $ls_T = $text_F->Scrolled('Text', -scrollbars => 'osoe', -wrap => '
+none');
$ls_T->pack(-fill => 'both', -expand => 1);

my $out_win = $ls_T;
my $tid;
my $cmd = 'ls -al 2>&1';
my $repeat = 6;
my $delay = 100;

$tid = $mw->repeat($delay => [\&update_ls, \$out_win, \$tid, \$cmd, \$
+repeat]);

MainLoop;

{
    my $updates = 0;

    sub update_ls {
        my ($text_ref, $tid_ref, $cmd_ref, $repeat_ref) = @_;

        $$text_ref->insert(end => scalar(localtime) . "\n" . qx{$$cmd_
+ref});

        if (++$updates >= $$repeat_ref) {
            my $times_format = '(Usr: %d, Sys: %d, ChUsr: %d, ChSys: %
+d)';
            $$text_ref->insert(end => sprintf $times_format => times);
            $$tid_ref->cancel;
        }

        $$text_ref->yview('end');

        return;
    }
}
[download]

Notes:

• From Tk::after, I've used the repeat() method to set it up; and the cancel() method when the callback's been called 6 times. [Beyond mirroring the "for my $i ( 0 .. 5) ..." (in your posted code), the 6 is not special.]
• Notice how all the callback arguments are references. This gives you a lot more flexibility: you could, for instance, change the delay, the command, or even the Tk::Text widget displaying the output.
• The defaults for -width and -height are 80 and 24, respectively: I've left those two out. I've made the -scrollbars optional (osoe).
• The code I've posted is complete, working and tested on a directory with 412 entries.

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