Perldoc Search: "$^P" perl-5.20.1
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10 : For example, whenever you call Perl's built-in "caller" function from the package "DB", the arguments that the corresponding stack frame was called with are copied to the @DB::args array. These mechanisms are enabled by calling Perl with the -d switch. Specifically, the following additional features are enabled (cf. "$^P" in perlvar):
36 : * When execution of the program uses "goto" to enter a non-XS subroutine and the 0x80 bit is set in $^P, a call to &DB::goto is made, with $DB::sub holding the name of the subroutine being entered.
1077 : $^P The internal variable for debugging support. The meanings of the various bits are subject to change, but currently indicate: