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/([perl]).*(?!\1)([perl]).*(?!\1)(?!\2)([perl]).*(?!\1)(?!\2)(?!\3)([p
+erl])/

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