As was already pointed out, you can get implication already with:

  (A and not B)
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(just look at the truth-table). As for your example, it can be cleanly written with:

  ($method ne "post") or ($contenttype eq "application/x-www-form-urle
+ncoded)
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since you are never falling through the or unless the method is "post".

In fact I find the form I wrote easier to follow than the version with the implication operator that you asked for. There does come a time when offering many logically equivalent ways to say the same thing lends to confusion, and I think this crosses that line. (As does "unless" and "until" quite often.)