my %seen;

for my $len (2 .. length $string)
{
    ++$seen{$1}, pos($string) -= length($1) - 1 while $string =~ /(\w{$len})/g;
}

##</code><code>##

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

##</code><code>##

(*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 read. In fact, (?!) gets optimised into (*FAIL) internally.

##</code><code>##

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