Actually, the same is true of zero:

% perl -wce 0
-e syntax OK
[download]

Yes, thank you, and also of undef and (), and of any list whose elements are the scalars already identified; e.g.:

% perl -wce '( 1, 0, undef, , "di foo", "ds bar", "ig baz" )'
-e syntax OK
[download]

Given dave_the_m's explanation above, is this to allow:

0 until ....
[download]

??

Reading between the lines, I gather that you are attaching some significance to the boolean value of the constant in question. I did too on a first reading, but then I realized that it is the boolean value of what follows the while or until that matters. I.e.,

1 while some_condition_with_side_effects;
[download]

0 while some_condition_with_side_effects;
[download]