Chances are, there's a well-developed API in Perl for your external system. Even if there isn't an API now (eg, for a home-made system), Perl is a sufficiently general-purpose language that it may be very easy to make a new one. Try searching with key words appropriate for your system/application.

As for "learning and creating PERL APIs", the best book or online resource for you of course depends on your background and needs. You could give us a few details or check the Book Reviews on this site.