Yes, that is what it means. Any time you have stmt if cond, it can be rewritten as

if (cond) {
  stmt;
}
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So, in this case, that line is equivalent to

if (! defined $max || $_ > $max) {
  $max = $_;
}
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Personally, I prefer the method of initializing $max to the first (defined) item in the list so as to avoid testing !defined $max on every pass. The performance benefit of avoiding that test is negligible, of course, but it reads more cleanly to me when you're able to just say "remember this as the highest value if it's greater than the previous highest value" without the extra "and a highest value has been previously set" clause.