=over

=item $PERLDB

=item $^P
X<$^P> X<$PERLDB>

The internal variable for debugging support.  The meanings of the
various bits are subject to change, but currently indicate:

=over 6

=item 0x01

Debug subroutine enter/exit.

=item 0x02

Line-by-line debugging.  Causes C<DB::DB()> subroutine to be called for
each statement executed.  Also causes saving source code lines (like
0x400).

=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.

=item 0x40

Use subroutine address instead of name when reporting.

=item 0x80

Report C<goto &subroutine> as well.

=item 0x100

Provide informative "file" names for evals based on the place they were compiled.

=item 0x200

Provide informative names to anonymous subroutines based on the place they
were compiled.

=item 0x400

Save source code lines into C<@{"_<$filename"}>.

=item 0x800

When saving source, include evals that generate no subroutines.

=item 0x1000

When saving source, include source that did not compile.

=back

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 L<perldebguts>.

=back