Hi singho,
There are several things wrong here.
1. According to your post title, do you what the 5th Element of the whole file, or each line in the file?
If your intention is the 5th Element of the whole file, then do this:

my @string;

open(SYSLOG, "/tmp/log100lines") or die "Can't open file: $!\n";

foreach my $line (<SYSLOG>) {
        chomp $line;
        push @string, $line
}

  print $string[4],$/;
[download]

2. But it seem, to me you want the 5th element of each line, which in that case you need do this:

open(SYSLOG, "/tmp/log100lines") or die "Can't open file: $!\n";

foreach my $line (<SYSLOG>) {
        chomp $line;
        my @string = split(/ /, $line);  ## remove the function join
        print "$string[4]\n";
}
[download]

NOTE:
a.) The scope of the array variable "@string" in the both cases,
b.) In the second script, you don't need the function join, because it Joins the separate strings of LIST into a single string with fields separated by the value of EXPR which in your case is ',', so your 5th element of each line has an uninitialized value.

That been said, there are a number of things, I think one should take note:

1. use these pragma: warnings and strict
2. use lexically scoped file-handles and use 3 -argument open function like

   open my $fh, '<', "file_to_open" or die "can't open file: $!";
   [download]

3. use a while loop, to walk through a file, step-wise like:

   while(<$fh>){
    ...
   }
   [download]

4. close all opened file handles
5. Please, also check your shebang line