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

my $a_counter;
my $b_counter;

for my $string ( @strings )
{
    print "In $string there were $a_counter 'a's and $b_counter 'b's.\n" if ( $string =~ /(a(?{$a_counter ++; }  ))+(b(?{$b_counter ++;}))+/ );
}

##</code><code>##

In aaabbbb there were 3 'a's and 4 'b's.
In ab there were 4 'a's and 5 'b's.
In abb there were 5 'a's and 7 'b's.
In aabb there were 7 'a's and 9 'b's.
In aaabb there were 10 'a's and 11 'b's.
In aabbb there were 12 'a's and 14 'b's.