You need to set the mime type , use something like this:

 
use Mail::Mailer;
my $mailer = Mail::Mailer->new("sendmail");
   $mailer->open({ From    => '$you',
                   To      => $mailto,
                   Subject => 'foo',
                   'Content-type'=> 'text/html'
                 }) or warn "Can't open: $!\n";
    print $mailer $mail_string;
    $mailer->close();
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