#!perl -w
use strict;
use warnings;

use Win32::GUI;

my $DOS = Win32::GUI::GetPerlWindow();
Win32::GUI::Hide($DOS);

# Get text to diaply from the command line
my $text = defined($ARGV[0]) ? $ARGV[0] : "Software Update Notificatio
+n";

# Create a font to diaply the text
my $font = Win32::GUI::Font->new(
    -name => "Comic Sans MS", 
    -size => 24,
);

my $main = Win32::GUI::Window->new(
    -name => 'Main',
    -text => 'CTG Updates',
    -width => 300,
           -height => 200
);

# Add the text to a label in the window
my $label = $main->AddLabel(
    -text       => $text,
    -font       => $font,
    -foreground => 0x0000FF,
); 

my $icon = new Win32::GUI::Icon('GUIPERL.ICO');

my $ni = $main->AddNotifyIcon(
    -name => "NI",
    -id => 1,
    -icon => $icon,
           -tip => "CTG Updater"
);

$main->AddRadioButton(
    -name    => 'Default',
    -text    => 'Apply updates now',
    -default => 1,    # Give button darker border
    -ok      => 1,    # press 'Return' to click this button
    -width   => 125,
    -height  => 20,
    -left    => $main->ScaleWidth() - 200,
    -top     => $main->ScaleHeight() - 100,
);

$main->AddRadioButton(
    -name   => 'Cancel',
    -text   => 'Update Another time',
    -cancel => 1,    # press 'Esc' to click this button
    -width  => 125,
    -height => 20,
    -left   => $main->ScaleWidth() - 200,
    -top    => $main->ScaleHeight() - 80,
);
my $textfield = $main->AddTextfield(
    -name   => "Textfield",
    -text   => "have fun and more",
    -left   => 75,
    -top    => 150,
    -width  => 200,
    -height => 40,
    -readonly => 1,
  );
#Win32::GUI::Dialog();
#sleep 30;

$main->Show();
Win32::GUI::Dialog();
exit(0);

sub Main_Terminate {
    return -1;
}

sub Main_Minimize {
    $main->Disable();
    $main->Hide();
    return 1;
}

sub NI_Click {
    $main->Enable();
    $main->Show();
    return 1;
}

sub Default_Click {
    print "Default button clicked\n";
    $main->Hide();
    return 0;
}

sub Cancel_Click {
    print "Cancel button clicked\n";
    $main->Hide();
    return 0;
}
[download]

While it's possible that I'm seriously misunderstanding the issue, I do believe that it's completely unnecessary to catch the logout event because Windows already has this hook built-in. See this technet explanation. The links on that page offer more details.

In other words, your script will only be triggered on this event so there will never be a need to look for it -- it'll only run when appropriate.