Current state of the interpreter.

$^S         State
---------   -------------------------------------
undef       Parsing module, eval, or main program
true (1)    Executing an eval
false (0)   Otherwise

The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.

The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because the undef value does not indicate whether exceptions are being caught, since compilation of the main program does not catch exceptions.

This variable was added in Perl 5.004.