Lots of sage advice. Now for a brief summary:

• Closing a filehandle forces a flush of the buffer.
• Closing a filehandle releases the filehandle resource.
• Closing a filehandle allows for a final check to ensure that things worked as planned. (or die $!)
• Closing a filehandle improves code readability. It tells future readers "I'm done with that."
• Closing filehandles improves code portability to platforms such as mod_perl where perl never actually terminates.
• Closing filehandles reduces errors at a distance (This relates to the 'or die' list item).

Closing read-only filehandles doesn't really run the risk of buffering or 'final check' problems. But it does still relate to resource usage, portability, and readability.