Hi, I see the same error as you on both versions of the script. I'm on Linux with Perl 14.2.

I don't know the reason it is happening, but you can make a workaround by making your toplevel window reusable, inhibiting its Window Manager's Close icon, and packForgetting its old widgets. This works for me. ( the code could use some cleanup, but it shows the way) :-)

#!/usr/bin/perl -w
use Tk;
use Tk::Table;
use Tk::Gauge;
use strict;
use diagnostics;

my $hw; # make a reusable toplevel
        
my $mw = MainWindow->new;
$mw->geometry("75x75");
$mw->title("Gauge testing");

my $button = $mw->Button(-text => "Null", -command => \&button3_sub)->
+pack(-anchor=>'center');

my ($alpha,$alphaone,$beta,$betaone,$mh,$output_text,$gaugevar,$cur);
#our ($alpha,$alphaone,$beta,$betaone,$mh,$output_text,$gaugevar,$cur)
+;

sub button3_sub {
$mh= (35+(29-1)*6);
$cur=$mh;

do_Toplevel();
#my $hw = $mw -> Toplevel;

$hw ->geometry("650x450");
$hw ->title('Health Track (!!!WARNING:DO NOT EXIT THIS WINDOW UNTIL DO
+NE)');
my $hf = $hw -> Frame(-background => 'white')->pack(-ipadx => 250, -fi
+ll => 'both', -expand =>'yes');
my $left_frame = $hf -> Frame (-background=>'white')->pack(-side=>'lef
+t', -fill => 'y');
my $right_frame = $hf->Frame(-background=>'white')->pack(-ipadx=>150, 
+-fill =>'y', -anchor=>'n',-expand=>1);
         $betaone = $left_frame->Entry(-background => "green",)->pack(
+-anchor=>'nw');
        my $beta_button = $left_frame->Button(-text => "Alpha Null", -
+command => \&update_output)->pack(-anchor=>'nw');
         $alphaone = $left_frame->Entry(-background => "red",)->pack(-
+anchor=>'nw');
        my $alpha_button = $left_frame->Button(-text => "Alpha Null", 
+-command => \&update_output)->pack(-anchor=>'nw');
        $output_text = $left_frame->Text(-height=>5,-width=>15)->pack(
+-expand=>0,);


my $gauge= $right_frame->Gauge(
     -background       => 'white',
     -bands           =>[
                {
                -piecolor => 'green',
                -minimum=> 50,
                -maximum=> 100,
                -tag=> 'Healthy',
                },
                {
                -piecolor => 'yellow',
                -minimum=> 25,
                -maximum=> 50,
                -tag=> 'Bloodied',
                },
                {
                -piecolor => 'red',
                -minimum=> 0,
                -maximum=> 25,
                -tag=> 'Critical',
                }
                 ],
     -bandplace => 'underticks',
     -bandstyle=> 'pieslice',
     -bandwidth=> 0,
     -caption => 'Current Value',
     -captioncolor=> 'black',
     -extent               => -180, 
     -from                 => 0,
     -hubcolor             => 'black',
     -huboutline       => 'blue',
     -hubradius           => 15,
     -majortickinterval    => 10,
     -majorticklabelplace  =>'outside',
     -finetickinterval     => 1,
     -minortickinterval    => 5,
     -margin           =>40,
     -needles              => [
{
     -arrowshape => [ 12, 23, 6 ],
     -color      =>       'black',
     -command    =>         undef,
     -format     =>          '%d',
     -radius     =>           196,
     -showvalue  =>             1,
     -tag        =>            'null',
     -title      =>            'null',
     -titlecolor =>       'white',
     -titlefont  =>'Helvetica-12',
     -titleplace =>       'south',
     -variable   =>      \$gaugevar,
     -width      =>             5,
}
                              ] ,
     -start                => 180,
     -to                   => 100,
 )->pack(-fill=>'both',-expand=>1);

##my $gaugebutton1 = $left_frame ->Button(-text => "Update", -command 
+=> \&gaugebutton1_sub)->pack(-side=>'bottom'); #currently pointless. 
+will leave in for possible diagnostics.

}

sub gaugebutton1_sub {
$gaugevar=($cur/$mh)*100;
}#above sub redundant(?) due to &update_output#
sub update_output {
  my $beta = $betaone->get();
  my $alpja = $alphaone->get();
  if ($alpha eq "") { $alpha = 0; }
  if ($beta eq "") { $beta = 0; }
$cur=$cur+$beta-$alpha;
  if ($cur gt $mh){$cur=$mh;}
  if ($cur lt 0){$cur=0;}
  my $output = "Current value:\n$cur";
  $output_text->delete('0.0', 'end');
  $output_text->insert("end", $output);
  $betaone->delete('0.0', 'end');
  $alphaone->delete('0.0', 'end');
$gaugevar=($cur/$mh)*100;
}

MainLoop;
###end


sub do_Toplevel {
    if ( !Exists( $hw ) ) {
        $hw = $mw->Toplevel();
        
        $hw->protocol('WM_DELETE_WINDOW' => sub {}); #prevent WM close
+ button working
        
        $hw->title( "Toplevel" );
        $hw->Button(
            -text    => "Close",
            -command => sub { $hw->withdraw }
        )->pack;
    }
    else {
            #clear out old widgets    
          my @w = $hw->packSlaves;
              foreach (@w) { $_->packForget; }

            $hw->Button(
            -text    => "Close",
            -command => sub { $hw->withdraw }
        )->pack;
        
        
        $hw->deiconify();
        $hw->raise();
    }
}
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