Here are two options:

1. send the xml to the browser, and let the browser render it:

   use CGI qw(:cgi); # don't need HTML, only cgi stuff
   print header(-type => 'application/xml');  # make browser expect xml
   open XML, '<', 'foo.xml' or die $!;
   print while <XML>;
   

2. print an HTML rendering of your xml file:

   use CGI qw(:standard);
   sub escape_xml ($) {
     my $text = shift;
     $text =~ s/</&lt;/g;
     $text =~ s/ < >/&gt;/g;
     return $text;
   }
   print header, start_html('XML File');
   open XML, '<', 'foo.xml' or die $!;
   print "<pre>\n";
   print escape_xml($_) while <XML>;
   print "\n</pre>";
   

updated to fix the typo spotted by benizi (Thank you!)