As expected, this works

#!/usr/bin/perl -w

use strict;
my $x = 12;
my $y = 14;
my $scale = '22.5';
my ($xmax,$ymax,$input);
calc();
while(1) {
  print "Enter X,Y,Scale: ";
  $input = <>;
  chomp $input;
  last if $input =~ m/^\s*$/;
  ($x,$y,$scale) = split(/\s*,\s*/,$input);
  calc();
}

sub calc {
  $xmax = $x * $scale;
  $ymax = $y * $scale;
  print "$x, $y, $scale, $xmax, $ymax\n";
}
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So it's something that Tk is doing to cause the problem