this should get you started:

use strict;
use warnings;

use Expect;

my $timeout = 5;
my $password = "password";
my $shell_prompt = ']$ ';

my $exp = new Expect;
$exp->raw_pty(1); 
$exp= Expect->spawn("su")
    or die "Error calling su:  $!\n";
unless ($exp->expect(5,"Password: ")) { 
  die; # add error handling
};
$exp->send("$password\n");
unless ($exp->expect(5, $shell_prompt)) { 
  die; # add error handling
};
print "Login successful\n";
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