I guess my concern is why the invalid open returned true.

Because the (piped) open succeeded. It found and successfully ran the prince executable.

The problem is that you are attributing the failed close to the failure to find the non-existent file; which is wrong.

What you are attempting to close is the pipe to the prince executable; which fails because the executable closed its end of the pipe, when it terminated because it couldn't find the file.

Bottom line: You cannot close the pipe because it is already closed. The confusion arises because you are attributing the failed close to the failure of the file to exist; rather than the fact the the pipe was already closed.

That's completely wrong.

1. Closing one end of a pipe doesn't cause close on the other end to return an error.

$ perl -E'
   pipe($r, $w) or die $!;
   close($w) or die $!;
   close($r) or die $!;
   say "ok";
'
ok
[download]

2. Closing a pipe created by open(my $fh, '|-', ...) reads the process (calls wait). It's very appropriate to do this, whether the pipe is closed or not. Closing such a handle returns the same thing as system.

$ perl -E'
   open(my $fh, "|-", "perl", "-e", "exit 123") or die $!;
   close($fh);
   die("$!\n") if $? == -1;
   die("Signal ".( $? & 0x7F )."\n") if $? & 0x7F;
   die("Error ".( $? >> 8 )."\n") if $? >> 8;
   say "ok";
'
Error 123
[download]

Well, the print wasn't dying, because the close() test actually runs.

You mean in your updated new code? Because the code you posted doesn't die if print fails.

I guess my concern is why the invalid open returned true. Seems counterintuitive.

Um, if open succeeded, and print succeeded, then it must not have been an invalid open, so nothing counterintuitive there

Its very possible for a program to end after a successful open to it