$prompt_warning->destroy if ($warnings eq "we're ok now!"; is premature and doesn't work. It works better by calling "after" right before "show". Here's a dialog that disappears after 10000 milliseconds:

#!/usr/bin/perl

use strict;
use warnings;
use Tk;
use Tk::Dialog;

my $mw       = MainWindow->new;
my $warnings = "";

my $prompt_warning = $mw->Dialog(
    -title          => 'Warnings:',
    -text           => 'danger',
    -bitmap         => 'warning',
    -default_button => 'OK'
);

my $answer = readmap(0);
my %question;
for ( my $i = 1 ; $i < 256 ; $i = $i * 2 ) {
    unless ( ( $answer & $i ) == 0 ) {
        $warnings = $question{$answer};
        $prompt_warning->after(
            10000,
            sub {
                $prompt_warning->Exit;
            }
        );
        $prompt_warning->Show();
    }
}

sub readmap {
    print "What is readmap()?\n";
}
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