why does ... work correctly, even for a filename like "0" that evaluates to false in boolean context?

Because it really looks like this under the hood

opendir(D, "test");
while ( defined( $file = readdir(D) ) ) 
...
[download]

This is a DWIM perlism, nifty huh? I think it applies to any assignment in a while statement where the result of the assignment is evaluated for the condition, or something along those lines.
HTH

_________
broquaint