defined does pretty much exactly what the English word suggests: It tests whether the variable has a defined value, which is to say, whether any value has ever been assigned to the variable.

In this particular case, if(defined getpwnam($nUsername)) checks to see whether getpwnam returns a value at all:
- If it finds the user, it will return something and "something" is always defined, so the test evaluates as true.
- If it doesn't find the user, it will return nothing (not just an empty value or a false value, but the absence of a value - in Perl, this not-a-value is called undef, for "undefined"), and the test evaluates as false.