my @list = ('10.1.1.1', '10.1.1.2', '10.2.2.1', '10.2.2.2');
@list = sort intermix @list;

sub intermix
{ 
    return (
	    substr( $a, 0, rindex($a, '.'))
	    eq 
	    substr( $b, 0, rindex($b, '.'))
	    )
	? 1 : 0;
}