I know this doesn't answer your question, but using File::Basename::fileparse is a better (and cross-platform) way to parse paths:

#! /usr/bin/perl -w

use strict;
use File::Basename;


BEGIN {
   our ($prog, $path) = fileparse $0, '.pl';
   open STDERR, ">>$path/$prog.err" or die "invisible error";
   warn "$0 started ". localtime() . $/;
}
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