The internal variable for debugging support. The meanings of the various bits are subject to change, but currently indicate:
Debug subroutine enter/exit.
Line-by-line debugging. Causes DB::DB() subroutine to be called for each statement executed. Also causes saving source code lines (like 0x400).
Switch off optimizations.
Preserve more data for future interactive inspections.
Keep info about source lines on which a subroutine is defined.
Start with single-step on.
Use subroutine address instead of name when reporting.
Report goto &subroutine as well.
Provide informative "file" names for evals based on the place they were compiled.
Provide informative names to anonymous subroutines based on the place they were compiled.
Save source code lines into @{"_<$filename"}.
When saving source, include evals that generate no subroutines.
When saving source, include source that did not compile.
Some bits may be relevant at compile-time only, some at run-time only. This is a new mechanism and the details may change. See also perldebguts.