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;

[download]

In reply to Re^2: need help to fix "use of uninitialized value..." by Marshall
in thread need help to fix "use of uninitialized value..." by mlebel

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.