HTML rendering ignores multiple whitespace and newlines and renders as a single space. You can either specify a Content-Type of text/plain or you can wrap your stuff in <PRE> tags which specify preformatted text ie fixed width, render the spaces as expected.

Given that you have the power of HTML to hand you could just do a nice table.

#!/usr/bin/perl -w
use strict;
print "Content-type: text/html\n\n"; 

my @rows = join '', map{"<td>$_</td>"}('router ip', 'sip','dip','sif',
+'dif','proto','sport','dport','pkts','size');
for (@sort_result) {
    my @tmpdata = split /\|/;
    push @rows, join '', map{"<td>$_</td>"} @tmpdata[0..7], $flows{$_}
+{pkts}, $flows{$_}{size};
}
@rows = map{ "<tr>$_</tr>\n" } @rows;
print "<table border=1>\n" , @rows, "</table>";
[download]

If you are outputing to a browser then as tachyon pointed out the multiple white space is going to get eaten during rendering. You would be better off using the intrinsic HTML methods of CGI.

One such implementation looks something like this:

use CGI qw(:all);
print header;
print start_html;
print "<table>\n"; # yuck... I hate this...
foreach my $row(@sorted_data){ # Don't know where @sorted data came fr
+om...
   print tr(map { td($_) } split(/|/,$row) );
}
print "</table>"; # end yuck
print end_html;
[download]

DISCLAIMER: this code is very untested and the author was very tired when he wrote it.
Hopefully there is enough there to at least give you an idea of how to proceed. You can sort out the rest I'm sure.

As the previous respondents have said, a html table may be a good solution.
However, depending on your desired effect, you might be happy with simply wrapping the text in a <pre> tag pair.

Without <pre> tags:

10.1.64.233 10.1.64.36 99.1.64.255 1 15137 137 1865 145956 0

With <pre> tags:

    10.1.64.233     10.1.64.36    99.1.64.255   1  15137   137  1865    145956         0

(I've only shown the numbers, because your header line has some strange inconsistencies which I think you still need to work out.)

print "Content-Type: text/plain\n\n";
[download]

It's easy to get all caught up in the fact that your CGI script has to print the content, forgetting the fact that your content can still be "marked up" with HTML tags.

So the fact is that your question is a HTML question. Because all that Perl has to do with it is that you're using Perl to output your HTML, and you just use simple print statements to do that. If you want Perl to output a centered header, you just say, "print "<center/><h2/>Header text!</h2></center>";"... pretty simple. The next trick is just learning HTML. I think that's a different website. ;)