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sub ext_s {        # Returns characters in sorted order
  my ($s) = @_ ;
...
  my @h ;
  return join('', grep { !$h[ord($_)]++ } split(//, $s)) ;
} ;

sub ext_dw {
  my ($s) = @_ ;
...
  $s =~ s/(.)/$h{$1}++?'':$1/ge ;
  return $s ;
} ;