At some place, the perl executable has to enter in contact with the operating system and invoke its open (or fopen?) system call, so that is a place to wedge your code in.

I don't know how your interception mechanism works, but if it tries to hand the program a fake libc instead of intercepting the system calls, you will run afoul of static linking. Whether your version of the perl executable uses static linking is another matter - Perl can be built to use static linking for some parts and dynamic linking for others. It may also well be that Perl uses some other call to open a file instead of open, but I don't know the C library well enough to suggest which other ones to intercept.

If you stay within Perl, you can always override GLOBAL::open, from C or from Perl. If you want to install this universally, put it in a Perl module and preload that module via the PERL5OPTS environment variable.

Previously I wrote:

This is interesting. I can intercept other functions like getpid and close, but there's something different about open.

I'd ask the perl5-porters mailing list - I'm sure someone there would know what's going on.

What's the XY Problem here? What are you really trying to accomplish? What do you have, and what are you trying to get? Perhaps there is a better way than trying to replace open.