Try like this:

#!usr/bin/perl -w
use strict;
use CGI;
my $res = "aaaaubbbbuooooo";
my @rest = split /u/,$res;
my $restLength = @rest;
print "Content-type: text/html\n\n";
print "restLength : $restLength\n";
for (my $i=0;$i< @rest;$i++){
   print "@rest[$i]\n";
}
__END__
restLength : 3
aaaa
bbbb
ooooo
[download]

length doesn't do what you want. You need scalar @array to get the size of an array.