try Perl_warn(...

I did. But have you ever looked at what lies behind that seemingly simple call? Debugging tools need to be simple and reliable, and Perl_warn() is neither.

When the simple act of adding the trace statement to a routine can cause it to go belly up, and the only way to track down why involves /E and wading through megabytes of post-processed C code to work out what actual code gets run, sticking to printf() is infinitely preferable.