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

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