Sub::Current makes available a function ROUTINE(), that returns a code
reference pointing at the currently executing subroutine.

In a special block (BEGIN, END, CHECK, INIT, and UNITCHECK in Perl 5.10)
this function will return undef.

Outside of a special block (that is, at the top level of a program)
ROUTINE() will return undef as well.

WWW: https://metacpan.org/release/Sub-Current