title: How does double arrow (=>) operator work in Perl?
body: <p>I know about the hash use of the <code>=&gt;</code> operator, <a href="http://www.tutorialspoint.com/perl/perl_hashes.htm" rel="noreferrer">like this</a></p>
  
  <pre><code>$ cat array.pl
  %ages = ('Martin' =&gt; 28,
           'Sharon' =&gt; 35,
           'Rikke' =&gt; 29,);
  
  print "Rikke is $ages{Rikke} years old\n";
  $ perl array.pl
  Rikke is 29 years old
  $
  </code></pre>
  
  <p>and I thought it was just syntax to initialize hashes, but in answers to <a href="https://stackoverflow.com/questions/4090424/how-can-i-qualify-a-variable-as-const-final-in-perl">How can I qualify a variable as const/final in Perl?</a>, <code>=&gt;</code> has been used like this</p>
  
  <pre><code>use Readonly;
  Readonly my $infilename =&gt; "input_56_12.txt";
  </code></pre>
  
  <p>What exactly does <code>=&gt;</code> mean? Are there more ways in which <code>=&gt;</code> can be used?</p>