Nice catch... :)

I can reproduce that on Win with 5.032001, but not the magic appearance of your

> 3.1     use feature 'say';

and the followin say statements, which are also not in the source you've shown.

I also looked into @DB::dbline which is aliased to @{"::_<zz.pl"} to be sure, and there was no "feature say".

Could it be you debugged another file, where you swapped print with say ?

FWIW: Those internal vars are explained in https://perldoc.perl.org/perldebguts#Debugger-Internal-Variables

update

> but it's pretty surprising that we've had this bug for 32 years!

Frankly, the debugger does so may weirdly elaborated things ...

... I assume most who saw this just thought it's a yet unknown feature to be explored.