in reply to sending a file to a perl script
perl AddSuffix.pl FileToAddSuffixTo
and the code can simply be:
... my $file = $ARGV; ...
@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.