The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module Fcntl. However, for historical reasons, some values are universal: zero means read-only, one means write-only, and two means read/write.

If the file named by FILENAME does not exist and the open call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If PERMS is omitted, the default value is 0666, which allows read and write for all. This default is reasonable: see umask.

The IO::File module provides a more object-oriented approach, if you're into that kind of thing.