As others have said, this isn't really a Perl issue, but what the heck.

Firstly, make sure each user has an appropriate umask (of, say, '002' instead of a common default of '022'). This ensures that the users write files allowing write permissions to their group.

Secondly, make sure Samba is running with a similar umask. This ensures files Samba creates are writable to the same group. Oh, make sure Samba is in the same group.

Thirdly, you may wish to make use of the setgid bit in directory permissions, changing the group ownerships of the directories appropriately:

$ ls -ld some_directory
drwxr-xr-x   2 user    group   ...
$ chmod 2775 some_directory
$ ls -ld some_directory
drwxrwsr-x   2 user    group   ...
[download]

This ensure that any files or directories created under that directory will automatically be owned by the appropriate group. If all users writing files to these directories are set up with the same GID, though, this is probably redundant.