OK once again...

( supposing that the numerical order of IDs is not the compare order you want, but that you still need the ID for further evaluation)

do this to keep record of the order of the keys

my %order;
while ( my $rec = $data->fetchrow_hashref ) {
    push @{ $result{ $rec->{"ID"} } }, $rec->{"item"};
    push @order, $rec->{ID};                           # read order
}
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and do this to compare the neighboring keys

my $last = $order[0];                                  # first key

for my $cur ( @order[1..$#order] ) {                   # following key
+s
   compare($last,$cur);
   $last = $cur;                                       # keep current 
+as last
}
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plz note: compare() is a sub you have to provide, you said "this i know how to".

Within compare you can access the values of the hash '%result' with the neighboring keys '$last' and '$cur'.

like

sub compare {
  my ($previous,$present) = @_;
  ...
}
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