At you. moritz provided the fix. In addition to moritz's correction, you can also clean up your regex by eliminating the extraneous parentheses, which just serve to obfuscate what is going on:

use strict;
use warnings;
use 5.010;

my @strings = (
    'aa',
    'ab',
    'a1',
);

for my $str (@strings) {
    if ($str =~ /
                    ([a-z])    
                    (?!\1)     
                    [a-z]  
                /x   ) {
        say "$str: $1";
    }
}

--output:--
ab: a
[download]

Thanks.