gv.c
457-        else if (topgv && GvREFCNT(topgv) == 1) {
458-            /* cache the fact that the method is not defined */
459:            GvCVGEN(topgv) = PL_sub_generation;

...

1433-       /* Adding a new name to a subroutine invalidates method cache */
1434:       PL_sub_generation++;