You need to be careful about spawning a toplevel in a sub with widgets added, because you will gain some memory for each invocation. It can be done, but Tk is not too good about cleaning up after itself, especially if secondary widgets( the label) are involved. It involves a thing called the ref-count, and has been discussed many times here in more advanced questions about unwanted memory gains in Tk programs. It is best to create 1 toplevel, and control it thru withdraw, and deiconify and raise. Here is one good way.

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

my $tl;
my $mw = MainWindow->new;
$mw->title( "MainWindow" );

my $spawn_button = $mw->Button( -text => "Toplevel", 
                -command => \&do_Toplevel )->pack();

MainLoop;

sub do_Toplevel {
        $spawn_button->configure(-state=>'disabled');

    if ( !Exists( $tl ) ) {
        $tl = $mw->Toplevel();
        $tl->geometry('300x100+100+100');
        $tl->title( "Toplevel" );
        $tl->Button(
            -text    => "Close",
            -command => sub { $tl->withdraw; 
                 $spawn_button->configure(-state=>'normal');
                 }
        )->pack;
    }
    else {
        $tl->deiconify();
        $tl->raise();
    }
}
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