=over

=item $PERLDB

=item $^P

The internal variable for debugging support.  Different bits mean the
following (subject to change): 

=over 6

=item 0x01

Debug subroutine enter/exit.

=item 0x02

Line-by-line debugging.

=item 0x04

Switch off optimizations.

=item 0x08

Preserve more data for future interactive inspections.

=item 0x10

Keep info about source lines on which a subroutine is defined.

=item 0x20

Start with single-step on.

=back

Note that some bits may be relevent at compile-time only, some at
run-time only. This is a new mechanism and the details may change.

=back