As I already said "you can get run-time warnings but not compile-time warnings for code entered at the debuger's command prompt," which I think answers part of your question. And I doubt there are any docs that cover this.

As for the "why", I assume that the debugger restores $^W in-line with the eval of your code via the equivalent of eval "\$^W= $saved; $your_code" (it actually restores quite a few things so the syntax isn't quite what I wrote) which means that $^W isn't restored until after that code is compiled. Patching the debugger to do eval "BEGIN{$^W= saved} $your_code" would correct that but you'll have to be careful to separate the things that need to be restored at compile time from the things that can't be restored from within the BEGIN block because the restoration would be ruined when you leave the scope of that block.

Creating such a patch sounds like a reasonable project. Let me know if you run into problems. ;)

        - tye (but my friends call me "Tye")