I'd like to use it in a larger script to log STDERR to a file.

You don't need a seperate program for doing that. You can just reopen STDERR to wherever you like:

open(STDERR, '>', '/path/to/logfile') 
    or die "Can't open STDERR to /path/to/logfile: $!\n";
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open(STDERR, '>error.txt');
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I can't just redirect it, because I also want to see it at runtime.

---
A fair fight is a sign of poor planning.

You could probably do some tricks with IO::Multiplex. Just save the orginal filehandle like this:

my $old_stderr = *STDERR;
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And then reopen STDERR with IO::Multiplex with $old_stderr and a filehandle to the log file.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

: () { :|:& };:

Note: All code is untested, unless otherwise stated

How about

use IO::Tee;
open(ERRLOG, '>error.txt');
my $tee = new IO::Tee(\*STDOUT, \*ERRLOG);
*STDERR = $tee;
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- Tom