Other monks have mentioned about error handling when the file to view is not found, so I will not say it again. Just a few comments on your code -

  my $request=$ENV{'QUERY_STRING'};
  $request=~/([-\w_]+\.($ext))/g;
  my $file=$1;
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can be written in one line as follows:

  my ($file) = $ENV{'QUERY_STRING'} =~ /([-\w_]+\.($ext))/g;
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And also the in code -

  foreach(@code){
    $_=~s/</&lt;/g;
    $_=~s/>/&gt;/g;
  }
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You can drop the $_ =~ part as it is not necessary (to save a few keystrokes)

  foreach(@code){
    s/</&lt;/g;
    s/>/&gt;/g;
  }
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If you want to improve your Perl programming skills, you should learn to use CGI and HTML::Template as these powerful modules can make CGI programming really easy.

Suggestion: Why don't you go off and modify your script to use CGI, and then post your new code here as part of a learning experience?