Am I doing something wrong?

You're giving a description of your code, instead of giving actual code that shows the error

The error you get is simple/easy to understand, but a description of code isn't going to demonstrate the error

For example

$ perl -e" Demo(); "
Undefined subroutine &main::Demo called at -e line 1.

$ perl -e" sub demo {warn 666} Demo(); "
Undefined subroutine &main::Demo called at -e line 1.

$ perl -e" package Demo;sub Demo {warn 666}; package main; Demo(); "
Undefined subroutine &main::Demo called at -e line 1.
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The problem is obvious, first there is no Demo, then there is no Demo ("demo" is not "Demo"), then there is no Demo again