Both approaches are imperfect. But ultimately, I consider the info you can retrieve via lsof more useful than what you might find when digging around in the symbol table. The two main issues with the symbol table are:

• the file descriptor number in question may not be the one you're looking for (see my other reply below)
• the dup(2) may not have left any traces in Perl's symbol table at all (as I tried to point out above), in case the dup happened via some C-level code in an XS module.