title: how can I parse a xml file
body: <p>I am trying to parse a XML file. I download the data from here 
  <a href="http://mips.helmholtz-muenchen.de/proj/ppi/" rel="nofollow noreferrer">http://mips.helmholtz-muenchen.de/proj/ppi/</a></p>
  
  <p>and I use this code but I get error </p>
  
  <pre><code>use strict;
  use warnings;
  use XML::Twig;
  
  my $MIPS_file = $ARGV[0];
  my $xml = XML::Twig-&gt;new();
  my $data = $xml-&gt;XMLin("$MIPS_file");
  my $intList = $data-&gt;{'entry'}-&gt;{'interactionList'}-&gt;{'interaction'};
  foreach my $int (@{$intList}) {
    my $experiment_type = $int-&gt;{'experimentList'}-&gt;{'experimentDescription'}-&gt;{'interactionDetection'}-&gt;{'names'}-&gt;{'shortLabel'};
    my $partList = $int-&gt;{'participantList'}-&gt;{'proteinParticipant'};
    my ($p1,$p2);
    foreach my $protPart(@{$partList}) {
        if ($protPart-&gt;{'proteinInteractor'}-&gt;{'organism'}-&gt;{'ncbiTaxId'} eq "9606") { # select human proteins
      if (!$p1) {
        $p1 = $protPart-&gt;{'proteinInteractor'}-&gt;{'xref'}-&gt;{'primaryRef'}-&gt;{'id'};
      }
      else {
        $p2 = $protPart-&gt;{'proteinInteractor'}-&gt;{'xref'}-&gt;{'primaryRef'}-&gt;{'id'};
      }
        }
    }
    print "$p1\$p2\n";
  }
  </code></pre>
  
  <p>I put the file on a folder in desktop (mac) Then I open terminal and I invoke the program like <code>perl myfile.pl</code></p>
  
  <p>This is the error I get</p>
  
  <blockquote>
    <p>Can't locate XML/Simple.pm in @INC (@INC contains: /Users/admin/perl5/perlbrew/perls/perl-5.16.0/lib/site_perl/5.16.0/darwin-2level /Users/admin/perl5/perlbrew/perls/perl-5.16.0/lib/site_perl/5.16.0 /Users/admin/perl5/perlbrew/perls/perl-5.16.0/lib/5.16.0/darwin-2level /Users/admin/perl5/perlbrew/perls/perl-5.16.0/lib/5.16.0 .) at myfile.pl line 3.
    BEGIN failed--compilation aborted at myfile.pl line 3.</p>
  </blockquote>
  
  <p>After installing twig , now i get this error </p>
  
  <pre><code>Use of uninitialized value $MIPS_file in string at myfile.pl line 7.
  Can't locate object method "XMLin" via package "XML::Twig" at myfile.pl line 7.
  </code></pre>