my $cgi = CGI->new();
my $a = $cgi->param('a');
print $cgi->header('text/html; charset=utf-8');

use Encode;
my $a_iso = $a;
Encode::from_to($a_iso, 'utf8','iso-8859-1');

if ($a_iso =~ /^\p{L}\p{L}$/g)
{
print "ok"; # funktioniert mit umlauten..
}