Due to ForceArray=>1 the $xml->{'dut'} is an ARRAY ref, not a HASH ref! Therefore you should treat it as an array ref:

push @{$xml->{dut}}, {
  ipaddress => $ipaddress,
  macaddress => $Macaddress,
  model => $ProdShortName,
  frimware => $Firmware_Version, # I think you mean firmware, not frim
+ware
};
[download]

Try to print the $xml with Data::Dumper before you attemptto modify it to see how does the datastructure actually look.