On unix the command line is parsed by the shell first, so your program only sees the "input.txt" part, and your perl code only has the file handle. A recent discussion in the ChatterBox on the subject brought up the lsof Unix command, but I wouldn't call it convenient, and usually needs root permissions.

Another possibilty is to use stat to get the Inode number and then search the filesystem for the corresponding file. I don't know if that works for multiple file-systems, though.

Can't you rewrite your program as taking 2 filenames as arguments? It seems the most convenient by far.

J.