The current value of the debugging flags. May be read or set. Like its command-line equivalent, you can use numeric or symbolic values, e.g. $^D = 10 or $^D = "st". See "-Dnumber" in perlrun and "Tracing the interpreter execution" in perlguts.
The contents of this variable also affects the debugger operation. See "Debugger Internals" in perldebguts.
Mnemonic: value of -D switch.