If you're in +n.x you can find this with:

which nzbget
[download]

let's say it tells you it's in /usr/local/bin/nzbget you would modify your code to be:

    my @args = ("/usr/local/bin/nzbget", $item);
    system(@args) == 0
        or die "Could not run '@args' ($?): $!\n";
[download]

The ($?) will show you the return code that the program failed with (in case there was nothing useful in $!).

Watch out for the fact that system is a little different from other functions that you expect to return 1 on success such as open:

my $args = "/usr/local/bin/nzbget $item"
open("$args |") || die "Can't run '$args': $!\n";
[download]

If you're in +n.x

I don't think he is. Did you read his post closely? He doesn't actually say it, but it's pretty obvious he's on Windows. First, he changes the case on the name of his executable from one usage to the next, betraying an assumption that the filesystem is not case-sensitive, which pretty much precludes any POSIX system. His title also has System in title case, further evidence he's not thinking like a *nix user.

Second, the wording where he says that the file is executable strongly reminds me of the way DOS and Windows users state such things ("is an executable file"), not the way *nix people usually say it ("file is executable by the user" or "file has execute permission" or such). This is perhaps subtle, but the wording makes me think, "the filename has .exe on the end of it", not "chmod +x".

I believe you mentioned in the CB that you solved this problem. ("sorry, I just figured it out. The variable is not being passed properly.") I bet it's because you didn't properly quote and escape the contents of $item. I suggest that you use substitute
system("nzbget $item")
with
system('nzbget', $item)
It's safer since a shell isn't loaded, and you don't have to escape the contents of $item.