How to I print to STDERR only if STDOUT is a different destination?

On Unix-style systems, you should be able to do:

my @stat_err = stat STDERR;my @stat_out = stat STDOUT;my $stderr_is_not_stdout = (($stat_err[0] != $stat_out[0]) ||                            ($stat_err[1] != $stat_out[1]));

But that won't work on Windows, which doesn't have real inode numbers. It gives both false positives (thinks they're different when they aren't) and false negatives (thinks they're the same when they aren't).

You can do that (almost) with -t:

-t STDERR

will be true if it is a terminal, and likewise for STDOUT.

This still would not tell you what terminal, and if you redirect to the same file, you may stilll get both.

Hence, if

-t STDERR && ! (-t STDOUT) || -t STDOUT && !(-t STDERR)

or shorter

-t STDOUT ^ -t STDERR  # thanks to @mob

you know you're okay.

EDIT: Solutions for the case that both STDERR and STDOUT are regular files:

Tom Christianson suggested to stat and compare the dev and ino fields. This will work in UNIX, but, as @cjm pointed out, not in Windows.

If you can guarantee that no other program will write to the file, you could do the following both in Windows and UNIX:

1. check the position the file descriptors for STDOUT and STDERR are at, ifthey are not equal, you redirected one of them with >> to anonempty file.
2. Otherwise, write 42 bytes to file descriptor 2
3. Seek to the end of file descriptor 1. If it is 42 more than before, chances are high that both are redirected to the same file. If it is unchanged, files are different. If it is changed, but not by 42, someone else is writing there, all bets are off (but then, you're not in Windows, so the stat method will work).