If the point of the OP's exercise is to avoid exposure of the perl source code, then storing the decrypted program as a file anywhere in the user's disk space would defeat the purpose.

Having the script change its "name" in the process table (by setting $0) would not be sufficient protection. Anyone savvy enough to figure out that a perl script is being run from a disk file can search the disk for recently created files and potentially find the perl source code. So the point is to avoid saving the program to a disk file.

UPDATE: Having said that, of course, there is a work-around that might supply some level of protection:

• The C program decrypts the perl script, stores the clear-text source code as a file, and issues a system call to execute the script.

• The perl script begins with  unlink $0; -- at the point where this step is executed, the script is already in memory, so the disk file is no longer needed. The exposure as a disk file lasts for just a fraction of a second.

Even with that, though, the sort of trick demonstrated by dave_the_m below can be tweaked to reveal the source code -- e.g. replace /usr/bin/perl with a shell script that launches the given perl script using the perl debugger. (I haven't tried it, but this or something similar is bound to be possible and not that hard to do.)

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