bigrat is lexically scoped , example

$ perl -E " { use bigrat; say 1/3+1/4; }; say 1/3+1/4; "
7/12
0.583333333333333
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perl5db.pl uses eval to execute code you type, and each eval is a new lexical scope

This is one reason I don't deal with debuggers

update: oh look, someone ran into this limitation before, see Debugger, use strict, use warnings and Are debuggers good?

One idea, how to make lexical pragma global, hints ( pl_hints $^H %^H ), taint::all, but I kind of think any debugger shouldn't need help with this kind of stuff anyway, it should be a feature , but like I said, I don't use them