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my %seen;

...
{
    ++$seen{$1}, pos($string) -= length($1) - 1 while $string =~ /(\w{
+$len})/g;
}

----------------------------------------------------------------------
  (?{$seen{$1}++})         run this block of Perl code
----------------------------------------------------------------------
  (?!)                     fail
----------------------------------------------------------------------

(*FAIL) (*F) 

This pattern matches nothing and always fails. It can be used to force
+ the engine to backtrack. It is equivalent to (?!), but easier to rea
+d. In fact, (?!) gets optimised into (*FAIL) internally.

$string =~ /(\w{2,})(?{$seen{$1}++})(?!)/;