..really weird :) Firstly I removed all stringified numbers, all STDOUT explicit mention and implicit hash element access.. not the cause of your errors but just to clean the field.

Then I suspect the problem being in the -textvariable behaviour: I suspect problem accessing elements of a nested datastructure. Infact uncommenting my DUMMY assignement I found sometimes val being undef (using dd to dump the whole hash)

#!/usr/bin/perl

use Data::Dump;

use strict;
use warnings;
use Tk;
use Tk::ProgressBar;

my %pid=(ex1=>{as=>'temp', val=> -21.5});

my $mw=MainWindow->new();
my $tmpbar=$mw->Frame()->pack( -padx=>30 );

my $dummy = 'DUMMY';
# my $dummy = $pid{ex1}->{val};
PBar( $tmpbar, \$dummy, -30, 70 )->pack();
# PBar( $tmpbar, \$pid{ex1}->{val}, -30, 70 )->pack();

$tmpbar->Label( -textvariable => \$pid{ex1}{val} )->pack();
$tmpbar->Label( -text => $pid{ex1}->{as} )->pack();

print  "val Labels: $pid{ex1}->{val}\n"; dd %pid;

MainLoop;

sub PBar {
    my($frame, $val, $min, $max)=@_;
    print  "val PBar1: $$val\n";
    my $pbar=$frame->ProgressBar(
        -anchor=>'s',
        -width=>100,
        -length=>500,
        -blocks=>100,
        -gap=>1,
        -resolution=>0.5,
        -variable=>$val,
        -from=>$min,
        -to=>$max
    );
    print  "val PBar2: $$val\n"; dd %pid;
    return $pbar;
}
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L*