I would take a look at HTTP LITE. It should be easy enough for Perl to download a web page, do a regexp scan for the files you're looking for, save that file to disk as index.html, and then start downloading all the other items you're looking for. Something like...

use HTTP::Lite;
my $http = new HTTP::Lite;
my $req = $http->request("http://www.something.com") 
        or die "Unable to get document: $!";

my $mirror_home = '/home/user/mirror_home/';
my (@javascript, @css, @jpg);
my $i = 0;

while ($http->body()){
  if ($_ =~ m/*.jpg/){ push $_, @jpg;}
  else if ($_ =~ m/*.js/){ push $_, @javascript;}
  else if ($_ =~ m/*.css/){ push $_, @css;}
}

open FILE, "> $mirror_home/index.html" 
  or die "Couldn't open $mirror_home/index.html : $!";

print FILE $http->body();

close FILE;

while ($i <= $#css){
  $req = $http->request("http://www.something.com/$css[$i]") 
        or die "Unable to get document: $!";   
  open FILE, "> $mirror_home/$css[$i]";  
  print FILE $http->body();
  close FILE;
 $i++
}

$i = 0;

# Then repeat for other extensions.
[download]

As a fair warning the above is definitely untested and probably horribly over-simplified, but the basic idea seems sound to me.