Devel::callsite module provides subroutines to get the caller return op address and perl interpreter context.

the callsite() function returns the op address of the caller, a number, one level up from where it was called. it's useful for functions that need to uniquely know where they were called, such as every::every(); see every. or it can be used to pinpoint a location with finer granularity than a line number (see http://www.perlmonks.com/?node_id=987268). in conjunction with an op tree disassembly you can know exactly where the caller is located in the perl source.

the context() function returns the interpreter context as a number. this is a fairly unique number together with the call site.