You didn't verify that either open succeeded. You didn't chomp the value you are using as a file name...

Very good points!

... so it probably failed!

Sorry, no - open: "The filename passed to the one- and two-argument forms of open will have leading and trailing whitespace deleted and normal redirection characters honored."

Even if the if the second open failed, it doesn't explain the warning, as the inner while loop would not execute - the same comment applies to LanX's post.