Interesting question, I'll be following the replies closely.

I use overloading and tie-ing a little, more often to add extra functionality while still looking core-ish.
Examples: exception stringification and bool overloading or using IxHash for ordering.

I've also dabbled in source filters, once to create debugging macros and I also started a Literate Pod filter.

I'm reluctant to override core functions, add to UNIVERSAL, globally change special variables, etc for fear of breaking some remote piece of code.

Some times I (mis)use core perl when I can use a certain aspect of it's behaviour even though it is outside the original intention. Eg. using @ISA, isa() and can() on hierarchies that aren't code classes. I love the fact that I can do this, that the language doesn't hide it away for my own good.

That's all I can think of at the moment.

Brad