A very good point!

Note also that in deeply nested hashes, exists will autovivify intermediate levels of the hash in the process of testing the existence of a low level element.

To be nitpicky, it's not exists, but the hash accesses preceding the exists call.

Anyway, I just wanted to point out that the ugly-but-entirely-core way to avoid the autovivification in this example is:

print 'yes'
    if exists $hash{www}
    && exists $hash{www}{xxx}
    && exists $hash{www}{xxx}{yyy}
    && exists $hash{www}{xxx}{yyy}{zzz};
[download]

Although as I described in a recent thread, I try to keep my hash accesses fairly simple.