I've made a few changes and I'm not sure if it works since I don't have any data to test it on...

#!/usr/bin/perl -w
use CGI::Carp qw(fatalsToBrowser);
use CGI;
use strict;

my $req = new CGI;
my $sort = $req->param("sort");
if (!defined $sort){ $sort = "id"; }

print "Content-type: text/html\n\n";
print "Click on a header to sort.<br>";
print "<table><tr><td><a href=\"?sort=id\">ID</a></td>\n<td><a href=\"
+?sort=info\">Info</a></td>\n<td><a href=\"?sort=graphics\">Graphics</
+a></td>\n<td><a href=\"?sort=friendly\">Friendly</a></td>\n<td><a hre
+f=\"?sort=techno\">Techno</a></td>\n<td><a href=\"?sort=entertain\">E
+ntertain</a></td>\n<td><a href=\"?sort=average\">Average</a></td>\n<t
+d><a href=\"?sort=hits\">Hits</a></td>\n</tr>";

opendir(DIR, "members") or die "Can't open the members directory, $!";
my @files = grep( /\.dat/, readdir(DIR) );
@files = sort(@files);
closedir(DIR);

my @all_hashes;
my $num = 0;

foreach my $file (@files){
  open (TXTFILE, "members/$file") or die "Can't open $file";
  my @lines = <TXTFILE>;
  close(TXTFILE);
  tr/\r\n//d for @lines;
  my $hits = $lines[0];
  if ($hits > 0) {
    my $info = sprintf("%.2f", ($lines[1]/$hits) );
    my $graphics = sprintf("%.2f", ($lines[2]/$hits) );
    my $friendly = sprintf("%.2f", ($lines[3]/$hits) );
    my $techno = sprintf("%.2f", ($lines[4]/$hits) );
    my $entertain = sprintf("%.2f", ($lines[5]/$hits) );
    my $average = sprintf("%.2f", (($info + $graphics + $friendly + $t
+echno + $entertain)/5) );
    my $id = $file;
    $id =~ s/\.dat//;
  
    $all_hashes[$num] = ({
       id        => $id,
       info      => $info,
       graphics  => $graphics,
       friendly  => $friendly,
       techno    => $techno,
       entertain => $entertain,
       average   => $average,
       hits      => $hits,
    });
  
  } # end if hits
  $num++;
} # end for each file

@all_hashes = sortby($sort, \@all_hashes);

for (@all_hashes){
  print "<tr><td>$_->{id}</td>\n<td>$_->{info}</td>\n<td>$_->{graphics
+}</td>\n<td>$_->{friendly}</td>\n<td>$_->{techno}</td>\n<td>$_->{ente
+rtain}</td>\n<td>$_->{average}</td>\n<td>$_->{hits}</td>\n</tr>";
}
print "</table>";

sub sortby {
  my $key = shift;
  my $aref = shift;
  my @array = @$aref;
  return sort { $a->{$key} <=> $b->{$key} } @array;
}
[download]

Update: You didn't have an 'average' or a 'hits' key in the hash so I did a search and replace and this what I ended up with.

Update 2: Now strict compliant.

Update 3: No more read-only value errors :-)