If you specify '<&=N', where N is a number, then Perl will 
do an equivalent of C's fdopen() of that file descriptor; 
this is more parsimonious of file descriptors. For example:

    open(FILEHANDLE, "<&=$fd")

open(FHAND,'>&=3') || die ("Dead as a dead thing");
[download]

Update

Changed < for > in the open as tye points out the original question was for writing not reading .. *dowt*
--

Zigster

Just what I was typing... Except he said "for writing" so replace the "<" with a ">".

        - tye (but my friends call me "Tye")

JP -- Alexander Widdlemouse undid his bellybutton and his bum dropped off --

For this program to have this requirement, it must be executing your script in such a way that the script inherits an open file descriptor 3.

If you test your script outside of this environment, then you will get the error you describe above. If you are testing inside the environment and get that error, then there is a good chance that the problem is with the other program.

        - tye (but my friends call me "Tye")

Alright. I get it.
Actually, I'm lying. I understand what you mean, but I guess I'm at a loss on how to make it do what I want.

When checkpassword is run, it checks fd 3 (Which, I believe, it supposed to be somehow already open - since the first line is a read()) and then returns an exit status depending on its result.
My script is the front-end to checkpassword, I need to open fd 3, write the username/pw pair to fd 3, and then run 'checkpassword' and watch its exit stat.

Any clues?

JP
-- Alexander Widdlemouse undid his bellybutton and his bum dropped off --