Each perl opcode is implemented by a pp_* routine, in this case pp_open() (in pp_sys.c).

In the latest sources, that eventually calls do_openn() in doio.c. That does a lot of preparatory work, and may hand off to one of various calls (eg, we may be opening a pipe, or duping an existing file handle), but in the simple case it hands off to PerlIO_openn() in perlio.c. What happens next depends on the defines with which your perl was compiled.

If you are using PERLIO, I think it then interrogates the layer stack for a layer that knows how to handle open, and then calls it. The layers consist of a table of function pointers, defined in perliol.h (and documented in pod/perliol.pod). I'm not sure where these tables are initialised.

I think in your case you are talking about the IPerlSys abstraction, mostly defined in iperlsys.h where I see PerlSIO_fopen() being defined in terms of the fopen() implementation you mention; that appears to be used only in PerlIOStdio_open() in perlio.c. However the 'IPerlStdIO' structure definition looks very similar to a PerlIO vector table, so I may be missing a point where it is used as one and its entry for pOpen being called directly.

I'm afraid I don't know enough about this area to give chapter and verse, but I hope I've given you enough information to help track down what you're actually looking for.

Hugo