As suggested above creating a new widget every tim ethe sub is called is not optimum. Try the following code with LEAK set to 1 and to 0 to see the difference in behaviour:

use warnings;
use strict;
use Tk;

use constant LEAK => 1;

my $var;
my $handle;

my $mw = MainWindow->new();

my $label = $mw->Label(-text=>"Subroutine Memory Leak")->pack;
my $button = $mw->Button(-text=>"ClickMe", -command=>\&startloop)->pac
+k;

#&startloop;
MainLoop;

sub startloop {
    my $iid;
    $iid = $mw->repeat( 300,\&autorefresh) ;
    return;
}

sub autorefresh {
    $var++;
    if (LEAK) {
        $mw->Label(-text=>"$var",-font=>'arial 14 bold')->place(-x =>1
+00,-y =>50,-anchor => 'nw')->pack;
    } else {
        $label->configure (-text=>$var);
    }
}
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