My PI doesn't want me to use any other modules other then the default ones that come installed with perl, so I can't use List::Util.

Firstly, List::Util is a core module, which means that it is one of "the default ones that come installed with perl". Feel free to use it, just as you are using Getopt::Std. Update: I didn't see ikegami's comment relating to this.

Secondly, even if it were not a core module, you could reuse some of the module's source code. So, tell your PI (whatever that is) to put that in his pipe and smoke it!