While you're waiting for the LogScale widget to show up on CPAN, you could experiment with offering the user a simple linear scale that controls the exponent to be applied to your avatar's body scale. Here's a stripped-down example of what the might look like:

#!/usr/bin/perl

use Tk;

$m=MainWindow->new(-title => "test");
$m->Button(-command => sub{exit(0)}, -text => "Bye")->pack;
my $scale_value_str = " current scale value =          ";
$m->Label(-textvariable => \$scale_value_str)->pack;
$m->Scale(-from => -2,
          -to => 2,
          -digits => 0,
          -resolution => -1,
          -command=>sub{$scale_value_str=sprintf " current scale = %8.
+3f ",10**$_[0]}
         )->pack;
MainLoop;
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(updated to show the resulting scale value in the Tk window, rather than on stdout)