Well, how many files do you have in that directory? hundreds? thousands?
The following may not be any faster but will certainly use less memory if you have a large number of files:

my ($dir, $file, $rnd) = ('/home/test/_html/index/', undef, undef);
opendir DIR, $dir or die $!;
while (1){
  $file = readdir DIR;
  last unless defined $file;
  next unless $file =~ /^index\.[^.]+\.html/;
  $rnd = $file;
  last if rand(10) > 5;
}
closedir DIR;
die "No files found\n" unless defined $rnd;
open IN, "<$rnd" or die $!;
print while (<IN>);
close IN;
[download]

If you want to make it faster, consider switching to mod_perl. There isn't much else you can do here.

--perlplexer