There's a definition issue on read_file but that is unused so I have commented it out to test it.

Yes, it's missing my $fn=shift; as the first line of read_file(). As I wrote: untested.

As I expected, but cannot explain, your code works as expected provided the GPIO is already exported. Your export code doesn't work and fails with Could not open /sys/class/gpio/gpio20/direction: Permission denied at afoken.pl line 12.

I have burried my spare Raspi unter tons of stuff, so I can't reproduce that at the moment. But I have two ideas why that happens:

• You are using a non-privileged account (i.e. not root), but /sys/class/gpio/*/* is writable only by root. Either run as root (bad) or change the permissions on sysfs. It seems the way to do so is udev (see https://unix.stackexchange.com/questions/68897/ and https://unix.stackexchange.com/questions/20125/). For tests, you also could work as root. But sooner or later, you will need to address the permission issue. On the other hand, if the Raspi just gets its time via NTP and does nothing else on the network, running as root is not that big issue.
• There may be a delay between export and actually creating the files in the sysfs filesystem, so your code may be too fast. Sleep for a few milliseconds after exporting and before accessing the new export.

I just had a look at the source code of the daemon mentioned in Re^12: Controlling USB on Raspberry Pi. It runs as root, so permission checks are effectively off. The source code does write a lot of debug output, so there is definitively a delay between exporting and setting direction.

Alexander