An example of davido's in-place string modification combined with boftx's dispatch table handler:

>perl -wMstrict -le
"use constant LEN => 5;
 ;;
 my $s = '1234598765555553456733333';
 print qq{'$s'};
 ;;
 my %dispatch = (
   '55555' => sub { return 'x' x length $_[0]; },
   );
 ;;
 for (my $offset = 0;  $offset < length $s;  $offset += LEN) {
   for (substr $s, $offset, LEN) {
     $_ = exists $dispatch{$_} ? $dispatch{$_}->($_) : $_ + 2;
     }
   }
 print qq{'$s'};
"
'1234598765555553456733333'
'1234798767xxxxx3456933335'
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Update: Changed example code to also exemplify topicalization of sub-string segment via for-structure (given no longer being quite kosher).