There are a few problems here. You are using %hash for 2 different things: a hash with key=site, value=ftp.siebel.com; a hashref with key={ftp.siebel.com}{ip_now} = 64.181.143.36 and {ip_new} = something other ip address.

Your loop structure complains that the hashref is missing when the key=site. Adding the following would get rid of the error, but only by filling in the void after line 39 above:

$hash{site} = $ftp_siebel_dns;
$hash{site}{ip_now} = "x";
$hash{site}{ip_new} = "y";
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A better way would be to change the structure to:

%HoH = (
  $ftp_siebel_dns => {
       ip_now => $ftp_siebel_ip,
       ip_new => $ftp_siebel,
       },
  );
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That looks about the same, but you would not have any key called site.