# Assuming your outside hash is named %h,
  print $h{main_data}[1]{AMOUNT} . "\n";

##</code><code>##

for (map {@$_} values %h){
  print "Account " . $_->{ACCOUNT} . " AMOUNT: " if exists $_->{ACCOUNT};
  next unless exists  $_->{AMOUNT};
  print   $_->{AMOUNT} . "\n";
}
# --- Output ---
# Account 6510334 AMOUNT: 200.00
# Account 6510334 AMOUNT: 0.00