However I am actually trying to capture STDERR, so this code isn't that useful to me.

You can't pass the file handles created by Tee modules to child processes since they don't correspond to system file handles, and neither is the file handle returned by open \$var.

You should probably start a new thread and state your actual problem. I could try to come up with an answer, but you'd benefit from starting over because 1) the question hasn't fully been revealed, and 2) you'd get more answers than just my own.