To be precise, a downward range is empty. For example,

print 0...1; print "\n";
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produces the result

01
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but inverting the 0 and 1 produces an empty line. z...a had no noticable effect -- still an empty line, but zzzzzzz...aaaaaaa did indeed cause perl to go into huddle while it was presumably calculating the combinations even though it should arguably know that it should promptly exit the range with an empty result when the second argument is lower than the first.

I consider this a missed optimisation opportunity in perl.