The other changes you suggested do prevent the error from happening.

No, it means an error never occurred in the first place.

What's setting $!?

Some earlier call or some internal call. You could use command line utility strace if you think it's relevant.

I do see how the "or die" on the join could be a logic error

Two problems:

• Your use of $! in the error message when it's not warranted.

• It does a bad job of detecting an error when one occurs. An error could occur and you wouldn't even know it. (False negative)

If you *also* want to make sure you received more than zero characters, that's fine. Add that check.

I was including $! in the error text because my Programming Perl, 3rd Edition book has the $! icon next to the readline function which is the implementation of the <FILEHANDLE> operator. If that's incorrect, then I'll omit it from the error message.

Does this pass muster:

my $error = '';
if (!eof($PROG_STDERR)) {
    $error = join('', <$PROG_STDERR>) or die "Error reading data from 
+STDERR";
}
close($PROG_STDERR);
print "error: $error\n";
[download]

You've given me some insight into the problem I've been fighting with. Thank you.

Does this pass muster:

When a reading error occurs, you don't say which error.

When the data received doesn't match the data you expected to receive, you incorrectly report a reading error (rather than a protocol error and specifying what you expected to receive).

Programming Perl, 3rd Edition book has the $! icon next to the readline function

readline does set $! when an error occurs. Unfortunately, it has no way of letting you know when an error occurred. That's where eof comes into play.

The docs for readline have been updated for 5.12.