>perl -wMstrict -le
"my $valid_string = qr{ \A [ACGT]+ \z }xms;
 ;;
 for my $user_subseq (qw(AAA AAC ACA CAA ABC)) {
   validate($user_subseq, $valid_string);
   my $n = 0;
   for my $string (qw(AAAAA  AAACACA  CAACAAA)) {
     $n +=()= $string =~ m{ (?= ($user_subseq)) }xmsg;
     }
   print qq{in all strings, '$user_subseq' = $n};
   }
 ;;
 sub validate {
   my ($user_supplied_string, $valid) = @_;
   return if $user_supplied_string =~ $valid;
   die qq{bad user supplied string '$user_supplied_string'}
   }
"
in all strings, 'AAA' = 5
in all strings, 'AAC' = 2
in all strings, 'ACA' = 3
in all strings, 'CAA' = 2
bad user supplied string 'ABC' at -e line 1.