in reply to B::C - Perl compiler's C backend

They are located in a shared library. You can get the full name of the library like this:

perl -MConfig -e 'print $Config{libperl}, "\n"'
Perl will search for the library in $Config{libspath}.

I don't know which operating system you are using but on Debian GNU/Linux you need to install the libperl-dev package to get the symlink needed for the perl C backend to work.