About "use header files from a pre-existing installation of Perl" - there exists a more general problem in BC++ compiler that they tend to name as "Borland include behaviour" in more recent compilers
A statement #include "file" has incorrect search order for header file.

More generally, perl-5.005-03 uses BCC version 5.02 and lower, but not 5.5. Support of 5.5 became in perls starting from 5.6.0 versions.

I also had some frustrated experience here, sent some corrective patches to p5p list, and sent bug reports to Borland.

As a side remark, I succeeded to use AS-built perl from within my Borland program, compiler version 5.5 .

Courage, the Cowardly Dog