Yeah it is really really cool
My requirements will complete if the parent or top level window hides or closed.
while the child window exists

... if you need it all to exit, without continuing with the same pid running, try this

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $notification = tkinit;
$notification->geometry('+400+400');

# here you can put stuff you want to occur when the window closes
# like send an email or something
#$notification->protocol('WM_DELETE_WINDOW' => sub {   });        

$notification->overrideredirect(1);    # Remove window decorations

$notification->Label(
    -text       => 'Your message displayed here for 5 seconds.',
    -foreground => 'white',
    -background => 'black',
    -height     => 5
)->pack;

$notification->after( 5000, sub{
    $notification->exit;
} );

MainLoop;
[download]

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I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku