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.