When I'm getting no output whatsoever to a webbrowser, the usual reason is that I've forgotten to print a header. Try

use strict;
use CGI::Pretty;
use Carp qw(croak);
my $sitelist_file='<c:\supportweb\content\sitelist.txt';
my $title = 'My Lines';
open my $data, '<', $sitelist_file
  or croak "No open on $sitelist_file";
my $text = do{local $/; <$data>};
close $data;
print header(),
      start_html({title=>$title});
foreach my $line (split "\n", $text){
  $response->write($line);
};
print end_html;
[download]

or even more compactly

use strict;
use CGI::Pretty;
use Perl6::Slurp;
print header(),
      start_html(-title=>'My Lines');
foreach (split "\n", slurp '<c:\supportweb\content\sitelist.txt'){
  $response->write($_)}
print end_html;
[download]

Not tested I'm interpreting you to mean that you're writing a cgi-bin program or some such. Unlike static webpages (.html pages) they need to have the magic

  Content-Type: text/html; charset=ISO-8859-1
[download]

or similar.

throop

hat-tip to Perl Best Practices

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• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.