Just to clarify, "Use of uninitialized value in string eq" is a warning (because you have wisely put "-w" on the first line of the script). It is not an error. It is simply telling you that at line 200 in file.pl, you are comparing two values with the "eq" operator, and one of them has not been initialized yet (it is "undef" at the point where that line of code is executed).

So is that really a problem? You don't actually say... is there something that the script is supposed to be doing that it does not do? Is it doing something (besides printing the warning) that it should not do?

You don't seem to be showing us line 200 of file.pl, so we don't know what's actually going on. It's not even clear whether it has anything at all to do with your DEFAULTIN file handle. (Maybe it isn't related, but if it is, maybe knowing what is at line 21 of "default.txt" would be relevant.)