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

my $input = "test\nfoo\nbar\n";
my $output;

{
   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");

   print while (<STDIN>);
}

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