Hi,
this was not your question, but instead of inserting spaces you can get centered lines by tagging them like so:

use Tk;
use strict;
use warnings;

my $mw = tkinit;

my $text = '';
$text .= 'x' x rand(40) ."\n" for (1..20);
my $t = $mw->Text()->pack;
$t->tagConfigure(centered => -justify => 'center');
$t->insert('end',$text,'centered');
MainLoop();
[download]

Cheers Christoph