Seeing how it is not documented

You could even say it's documented to behave differently than it does. Empty lists* evaluate to false, but while () {} loops endlessly.

$ perl -E'while (()) { say("foo"); last }'

$ perl -E'say("foo"), last while ();'

$ perl -E'while () { say("foo"); last }'
foo
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Intentionally or otherwise, it's definitely related to for (;;).

* — Parens don't create lists. Parens are sometimes required to indicate the presence of an empty list (e.g. "my $x = ;" and "= f();" are invalid), but not always (e.g. "print;" and "for () {}" are valid).