I don't have IO::Scalar installed, so I can't help you with that module. However, you can do it without IO::Scalar in 5.8.0:

# This \$var syntax of open() requires Perl 5.8.0 or higher.
use 5.8.0;


my $input  = "test\nfoo\nbar\n";
my $output = '';
my $code   = 'print while (<STDIN>)';

{
   local *STDIN;
   open(STDIN,  '<', \$input)
   or die("Can't open string for reading.\n");

   local *STDOUT;
   open(STDOUT, '>', \$output)
   or die("Can't open string for writing.\n");

   eval "$code";
}

print("\$output contains:\n$output");


__END__
output
======
$output contains:
test
foo
bar
[download]

A more general example:

# This \$var syntax of open() requires Perl 5.8.0 or higher.
use 5.8.0;

my $input = "test\nfoo\nbar\n";
open(my $old_stdin, "<&STDIN")
or die("Can't dup STDIN: $!.\n");
close(STDIN);
open(STDIN, '<', \$input)
or die("Can't open string for reading\n");

my $output = "";
open(my $old_stdout, ">&STDOUT")
or die("Can't dup STDOUT: $!.\n");
close(STDOUT);
open(STDOUT, '>', \$output)
or die("Can't open string for writing\n");

print($_) while (<STDIN>);

print $old_stdout ("\$output contains:\n$output");

__END__
output
======
$output contains:
test
foo
bar
[download]