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 ++;}))+/ );
}