I think you're wrong there. Postincrement only works different for strings if they start with a letter. For strings like "199" it makes no difference: you just get 200. And other strings apparently are converted to a number first, as $x = "2A"; $x++ produces 3.

$ perl -E '$x = $y = "A3"; $y + 0; $x++; $y++; say "$x $y"'
A4 1
$
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The difference is, $y has an integer (as well as a string value), where $x doesn't. And hence, the post-increment value differs.