Playing with groups.google.com I understand that <<Unix window managers do not have such a "Always on top" feature. To be more precise, most Unix window managers can do something like "Always on top", but there's no X11 standard.>> (from Slaven Rezic) From Slaven, the (untested) trick is: 
$top->bind("<Visibility>" => sub { $top->raise });

[download]

For win32, with a trick by Toby Everett, for me works this: 
use strict;
use Tk;
use Win32::API;

my $mw = new MainWindow;
use vars qw/$Button/;

$mw->Frame(-width=>200, height =>25);
$mw->update;
  Win32::API->new("user32","SetWindowPos",[qw(N N N N N N
N)],'N')->Call(hex($mw->frame()),-1,0,0,0,0,3);


$Button=$mw->Button(-text => "Button",
        -command => sub {
                &do_something;
            })
        ->pack();


MainLoop;

sub do_something {

    #sleep(10);
}

[download]

UPDATE:
Always from Slaven Rezic, there is a trick for KDE2:

If you're using KDE2 the you can write following ($w is your toplevel widget): 
    my($wrapper) = $w->toplevel->wrapper;
    $w->property('set', '_NET_WM_STATE', "ATOM", 32,
         ["_NET_WM_STATE_STAYS_ON_TOP"], $wrapper);

[download]
I think the _NET_WM_* stuff is also GNOME-compliant.

In reply to Re: stay-on-top property for Toplevel for Perl-Tk GUI by dree
in thread stay-on-top property for Toplevel for Perl-Tk GUI by Courage

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.