# untested
my $code = 'while (<>) { print; }'; # would typically be more interesting
my $output = ''; # storage for output
my $input = "more and more\nstuff here"; # would typically be more interesting

    {
     tie *STDIN, 'IO::Scalar', \$input;
     tie *STDOUT, 'IO::Scalar', \$output;
      eval "$code";
    }
# interesting stuff should be in output here