Description:

The _callsite()_ function returns the 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 the Every manpage. Or it can be used to the pinpoint
a location with finer granularity than a line
number|http://www.perlmonks.com/?node_id=987268 manpage. 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.