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$a = "a.com";
$b = "b.com";

if ($ENV{'SERVER_NAME'} =~/$a/) {
 print "location:http://www.a.com/a\n\n";
 exit;
}
elsif ($ENV{'SERVER_NAME'} =~/$b/) {
 print "location:http://www.b.com/b\n\n";
 exit;
}
else {
 print "location:http://www.c.com/a\n\n";
 exit;
}