I don't have IO::Scalar, but the following works. Downside: requires perl 5.8.0 or higher.

use 5.8.0;  # for open \$buf.
use strict;
use warnings;

use IO::Tee ();

my $stdout = '';

{
   my $fh_scalar;
   open($fh_scalar, '>', \$stdout)
   or die("Can't open \$stdout: $!\n");


   local *STDOUT = new IO::Tee(
      \*STDERR,
      $fh_scalar
   );


   print("Test\n");
}

print("\n");

print("\$stdout:\n");
print($stdout);
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I added local to easily restore the original STDOUT, but it's not needed.