Many have said how you should do it. But here are some tips in migrating what you already have, after adding use strict and -w.

First of all, use warnings; too. This is lexical and selective. When in use, that region of code ignores -w and does what you said instead. So, you can turn off specific warnings just around the passages of "old" code that have not been cleaned up yet.

Likewise with use strict;. You can start by simply heeding the text of the message and qualifying the name. Then, anything that doesn't pass but isn't immediatly fixable can be worked-around by using no strict 'refs'; or whatever type of strictness you need to turn off, just around the line or lines in question.

Then, after it's stable again, you can revisit each section of "no..." and alter the code bit by bit, maintaining a running version all along.

—John