.....any where on screen?.....try this for Tk

#!/usr/bin/perl -w
use strict;
use Tk;

my $mw = tkinit;

my $notification = $mw->Toplevel();
$notification->geometry('+400+400');
$notification->withdraw;               # Hide display window


$notification->protocol('WM_DELETE_WINDOW' => sub {   });        
#$notification->overrideredirect(1);    # Remove window decorations

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

$mw->Button(
    -text    => 'Display Notification Window',
    -command => \&display_note
)->pack;

MainLoop;

sub display_note {
    $notification->deiconify;
    $notification->raise;
    
    # this is where you setup a condition to withdraw it
    $notification->after( 5000, \&hide_note );
}

sub hide_note {
    $notification->withdraw;
}
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.... if you can use Gtk2, see Gtk2::StatusIcon Demo