while (<>)
{
  my $i=0;
  print join(" ",
    grep { (($i++) % 5) == 0 } 
    /\b(([perl])\w*(?!\2)([perl])\w*(?!\2)(?!\3)([perl])\w*(?!\2)(?!\3)(?!\4)([perl]))\b/g),"\n";
}