Redirecting stdin from within a script

[Kyle]

Seems like I used to be able to do this in a bash script (I'm running 
2.05b):

5<a_file

while read line; do
   echo $line
done <&5

But I'm getting the error: line 8: 5: Bad file descriptor

I need a script that will read from a file if a filename is specified or 
otherwise just use stdin (for the remainder of the script).

I've tried a number of things, like:

if [ "$fname" != "" ]; then
   0<$fname
fi

or

5<$fname
0<&5-

Is this a bug, or is there a way to get the file descriptor scope to 
extend throughout the rest of the script?

Kyle