my @strings = qw( abb aabbb aaabbbb ab aab aabb abbb );

for ( @strings )
{
    our $a_counter = 0;
    our $b_counter = 0;

    if ( /(a(?{$a_counter ++;}))+(b(?{$b_counter ++;}))+/ && $b_counter == $a_counter + 1 ) 
    {
        print "In '$_' there were $a_counter 'a's and $b_counter 'b's.\n";
    }
}