perl 里如何从一个字符串中一次读入一行，然后做匹配

use IO::File;
use strict;
use warnings;

my $str="aaaa\nbbbb";
open (HANDLE,"<",\$str);
while(<HANDLE>)
{
  if($_=~/a/ and $_!~/b/)//这可能有点笨
  {
    print($_);
  }
}

my $str="aaaa\naabb\nbbbb";
my @matched = grep /a/ && ! /b/, split "\n", $str;