Judging by your results on the warnings, or rather the lack of them, I think the subs return false, thus, !1. That is both equal to "" and numerically equal to 0 without a warning.

Oh, and a 1 is allowed to be used as a statement without warning. Typical use for them is in a bodyless loop:

1 while COND;
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Actually, a for loop does return something. Assuming it runs at least once, it will return an empty string, alias scalar false.

There seems to be no particular reason for that behaviour.

Makeshifts last the longest.

Actually, bart is saying it returns !1 which is really scalar false. It is subtly different than "" as can be seen here:

$ perl -wle 'print "yes" if !1 == 0 '
yes
$ perl -wle 'print "yes" if "" == 0 '
Argument "" isn't numeric in numeric eq (==) at -e line 1.
yes
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-sauoq
"My two cents aren't worth a dime.";

Oh, and another thing: you can use either 0 or 1 as a constant in void context without getting a warning, but you can also use any string that starts with "ig", "ds", or "di", as Dominus famously demonstrated.

Makeshifts last the longest.

as well as as the last statement in a required file/module.

Well, you can use whatever you want as the last statement of a required module without getting the "Useless use of a constant in void context" warning.

-sauoq
"My two cents aren't worth a dime.";

True, if you actually require the file. That probably is because then, the last statement isn't called in void context.

But you do get the warning if you try to test such a module for syntax errors using perl -wc FILE. Unless you actually use 1.