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$PERLDB
$^P

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

0x01

Debug subroutine enter/exit.

0x02

Line-by-line debugging.

0x04

Switch off optimizations.

0x08

Preserve more data for future interactive inspections.

0x10

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

0x20

Start with single-step on.

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