How about you show us an example were it doesn't capture the output? Here's one that shows it working.

#!/usr/bin/perl
use strict;
use IO::Scalar;
use Data::Dump qw(pp);
use Data::Dumper::Simple;

my $str;
my $io = tie *STDERR, 'IO::Scalar', \$str;
pp('monks');
undef $io;
untie *STDERR;

print Dumper($str);

__DATA__
Z:\Perl>ioscalar.pl
$str = '"monks"
';
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