I think you're coming up against a bit of perl DWIMery, that it decides a string "looks like a number" if it has only whitespace around the number. You can work around it by not doing that (eg by having "\n$amount -"), but you are best off just using printf and avoiding the (deprecated) $# variable:

  printf "\n%s - %2f = %.2f", $amount, $taxes, $amount - $taxes;
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