Well just call the program like:

perl AddSuffix.pl FileToAddSuffixTo
[download]

and the code can simply be:

...
my $file = $ARGV[0];
...
[download]

@ARGV is the special perl variable that holds the argument vector.

If all you're doing is changing the suffix (and not any reformatting, etc.), then might I suggest just renaming the file with the given .doc extension? If you're using windows you can just right click and go to rename (or the rename command in MS-DOS), and under UNIX, the 'mv' command would do it, I believe.