Your code is close but has two problems. First you do not use the g modifier in your regex, so your code runs into an endless loop. Second, your have a typo in this line:

$authentication .= $authentiction. $1. " ";
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That tells me you are using my but you are not using strict! Remember, always use strict.

If you change that code to

$authentication .= $authentication. $1. " ";
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all is fine.

That all boils down to

   while ($str =~/authentication ?(\S+)?/g){
      $authentication .= $authentiction. $1. " "; #more perlish
   }
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or even

  $authentication = join " ", $str =~ /authentication ?(\S+)?/g; #even
+ more perlish
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