As the title says, I'm not clear on when such a subroutine will be called. From the require page at perldoc one can write:

push @INC, \&my_sub;
sub my_sub {
   my ($coderef, $filename) = @_; # $coderef is \&my_sub
   ...
}

but where does this go exactly? The required package or the requiring script (or package)? I've tried both with some sentinel print statements but neither worked so clearly there is something I'm not getting.