Its a tkx bug plain and simple

See Tcl::Tk, this works

#!/usr/bin/perl --
use strict;
use warnings;
Main( @ARGV );
exit( 0 );
sub Main {
    Fudge();
    Fudge();
}
sub Fudge {
    use Tcl::Tk;
    my $int = new Tcl::Tk;
    my $mw = $int->mainwindow;
    my $lab = $mw->Label(-text => "Hello world")->pack;
    my $btn = $mw->Button(-text => "test", -command => sub {
        $lab->configure(-text=>"[". $lab->cget('-text')."]");
    })->pack;
    $int->MainLoop;
}
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Why this works? Because each $int Tcl::Tk is a new Tcl interpreter

Why the same doesn't work with Tkx? Because Tkx creates a single Tcl interpreter, and when it ends, that's it, it ends

You can patch Tkx.pm, its pure-perl, but this is a bug in Tkx

Thanks a lot for this explanation. i have to pass to Tk then.
juste a last question : why is it impossible to create a new Tcl Interpreter with tkx ?