Here's a slightly funny usage:

#!/usr/bin/perl
# file fusselkerl
use strict;
die "usage: $0 pattern [file]\n" unless @ARGV;
my $p;
{
  my (%s, %i);
  my $d = my $c = 1; # our regexp will be inside parens, so first back
+ref is 2
  $p = join (
    "",
    map {
      if($s{$_}++){
        "\\".$i{$_}
      }
      else{
        $i{$_}=++$c;
        $c>$d+1 ? '(?!'.join('|',map{"\\".abs}-$c+1..-$d-1).")(\\w)" :
+ "(\\w)";
      }
    } split //, shift
  );
}
print '(',$p,")\n";

local $" = ', ';
my %s;
while (<>) {
    my @l = ();
    while (/\b($p)\b/g) {
      push @l, $1 unless $s{$1}++;
    }
    print "@l\n" if @l;
}
[download]

try

perl fusselkerl fusselkerl /usr/share/dict/words
[download]

edit: oh, I posted that already years ago... Re: How to use a negative backreference in regex?, sorry ;)