I never heard about '_define' in perl (which is not to say it doesn't exist somewhere). Or did you mean just 'define' and used the underscore like I used the quotes?

If it is the latter case, then yes, you can define lots of things in perl, for example subroutines. Defining is exactly what you want us to do, i.e. you asked just now "Define 'define'". In a programming language 'defining' is giving the exact details, the implementation for a name, so that the computer knows what to do if that name comes up later

Anyway, where did you hear the word, in what connection?