TIMTOWTDI (unnecessarily verbose, but perhaps easy to follow and illustrating some possibilities with lookaheads):

#! /usr/bin/perl
use strict;
use warnings;

my $text = 'From: aaa@bb.netxxxMessage-Id: xxx<200902131530.n1DFUZ5l00
+1303@aaa.bb.net>xxxTo: xxx@yyy.netxxxSubject: Size atanan xxx numaral
+i ticket hakkindaxxxContent-Type: text/plain; charset=ISO-8859-9xxxCo
+ntent-Transfer-Encoding: 7bit';

if ( $text =~ /(?=To:\s)(.+)(?=[^x]{3})/ ) {
    $text =~ /.+To:\s(.*?)(?=xxxSubject)/;
    my $to_email = $1;
    print $to_email . "\n";
} else {
    print "no match\n";
}
[download]

Output:

xxx@yyy.net

as specified.

Your OP would have been easier to parse had you included data and code inside <c>...</c> tags as done above.