I believe an empty string, the undefined value, an empty list/array and the number zero (numeric or "0") will return a false value. A non-empty string, an array with 1 or more elements, or a non-zero number will return a true value.
in reply to What are all the false values in Perl ((conditional, boolean, whitespace, unprintable characters)
If you're seeing a zero, be sure it's not a string with other characters besides a zero in it. I believe spaces or other characters before or after it may cause the variable to be evaluated as a numeric zero when you're doing math, but since there's more than a number in the string, a boolean test will look at it from a string point of view, and will return 'true' since it's a non-empty string.
If $script::debug is being set to a numeric zero (or, better, undef), there's no reason the test you're describing should return a true value. I'd like to see how you're setting $script::debug.
(Updated 'list' vs. 'array'.)