You are running into a problem called "blocking the eventloop". The easiest way to solve it, is to run the &refresh_func in a thread. There are many details you must observe to use Tk with threads, and this assumes you have no Tk code in the refresh_func().

You can have a few shared variables, to pass information back and forth between your thread and main. The idea would be to let the thread update the network data as fast as it can, and if the user sets the refresh rate too low, he just keeps getting repeat data shown, until the thread is done. You can google for many "perl/tk threads" examples. Here is a an extra clever Tk Canvas threaded program, which uses Tk::Trace's tracevariable.

#!/usr/bin/perl
use warnings;
use strict;
use threads;
use threads::shared;

my $v:shared=0;
my $thread = threads->new( \&launch_thread )->detach;


package Tk;
use Tk::Trace;

package main;
use Tk;
use constant PI => 3.1415926;

my $mw = MainWindow->new;

$mw->fontCreate('medium',
   -family=>'courier',
   -weight=>'bold',
   -size=>int(-14*14/10));

my $c = $mw->Canvas(
              -width => 200,
          -height => 110,
          -bd => 2,
          -relief => 'sunken',
          -background => 'black',
          )->pack;
          
$c->createLine(100,100,10,100,
          -tags => ['needle'],
      -arrow => 'last',
      -width => 5,
      -fill => 'hotpink',
       );

my $gauge = $c->createArc(
        10,10, 190,190,
        -start => 0,
    -style => 'arc',
        -width => 5,
        -extent => 180,
    -outline => 'skyblue',
        -tags => ['gauge'],
);

my $hub = $c->createArc(
        90,95, 110,115,
        -start => 0,
        -extent => 180,
    -fill => 'lightgreen',
        -tags => ['hub'],
);


$mw->traceVariable(\$v, 'w' => [\&update_meter]);

$mw->bind('<Motion>' => sub{ $v += 1 });

$mw->repeat(50,sub{ $v-- });

my $text = $c->createText( 
      100,50,
       -text  => $v,
       -font  => 'medium',
       -fill  => 'yellow',
       -anchor => 's',
       -tags => ['text']
 );


$c->raise('needle','text');
$c->raise('hub','needle');

MainLoop;

sub update_meter {
   my($index, $value) = @_;
  if($value <= 0){$value = 0 }
  if($value >= 100){$value = 100}
    
    my $pos = $value / 100;
    my $x = 100.0 - 90.0 * (cos( $pos * PI ));
    my $y = 100.0 - 90.0 * (sin( $pos * PI ));
    $c->coords('needle', 100,100, $x, $y);
    $c->itemconfigure($text,-text => $value);
return $value;
}


sub launch_thread{

while(1){
  $v = 50 + int rand 50;
  print "$v\n";
  select(undef,undef,undef,.1);
 }

}
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