In Perl >5.10, there is a new operator //=. This checks for "definedness". $Pairs{$Person1}   //= $Person1; the hash value is assigned the value of $Person1 if that hash value wasn't already defined.

You cannot use an undefined value in a string concatenation. So, $PairsOf{$Person1} //= ""; will define $PairsOf{$Person1} to be a null string if it isn't already defined. If the value is already defined, then this should have no effect. A null string can be used in a concatenation.

      $Pairs{$Person1}   //= $Person1;
      $Pairs{$Person2}   //= $Person2;
      $PairsOf{$Person1} //= "";
      $PairsOf{$Person2} //= "";
      $PairsOf{$Person1}  = $PairsOf{$Person1} . " " . $Person2;
      $PairsOf{$Person2}  = $PairsOf{$Person2} . " " . $Person1;
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