If your on 5.8.1 (from memory), you can skip using IO::Scalar completely and using teh new form of open to a scalar (which means $/ is honoured correctly).

#! perl -slw
use strict;

my $x;
{
    local *STDERR;
    open STDERR, '>', \$x or die "STDERR $!";
    print STDERR 'stuff';
    warn 'Sommats up!';
}
print STDERR 'More stuff';
print "Caught: $x";
__END__

P:\test>381308
More stuff
Caught: stuff
Sommats up! at P:\test\381308.pl line 9.
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