I couldn't replicate the problem. I'm on FC6. Try this.

#!/usr/bin/perl

use strict;
use warnings;
use Tk;

my $mw   = MainWindow->new;
my $desc = $mw->Label(
    -text       => "SmartCheckin",
    -font       => [ -family => 'Courier', -size => 150, -weight => 'b
+old' ],
    -width      => 14,
    -anchor     => 'w',
    -relief     => 'groove',
    -background => 'grey',
)->pack;
my $entry  = $mw->Entry->pack;
my $button = $mw->Button(
    -text    => "SmartCheckin",
    -command => \&push_button
);
$button->pack;

MainLoop;

sub Entry {
    $entry->insert("SmartCheckin");
}

sub push_button {
    $entry->insert( 0, "Does this help? " );
}
[download]