This is actually for systems that don't support shebang execution.

If you look very carefully, you'll notice that the eval string is a shell command, not Perl. On systems that don't have shebang, this gets handed to a shell, which hopefully has Perl in its PATH. That shell then exec's perl -S script args where the name was $0 and the args, if any, came from the ${1+"$@"}.

Now, when it does get handed to Perl, the if 0 on the next line stops it from doing the eval (of course, the shell is line-oriented and never sees the if!) This would also happen if the system started supporting shebang execution and the file was handed to Perl directly in the first place.

Nasty but effective, no?