look a the documentation for open

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")

So

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

Should open a file handle to descriptor 3. If you really want more info on what exactly descriptors are then follow up, this should get you on your way.

Update

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