Why a hash ref rather than a hash? Surely it is simpler and clearer to write:

my %record;

while (<IN>) {
  if (/^(\d+).../) { # found beginning of new record
    process (\%record) if $record{id};
    %record = ();     # Flush old record
    $record{id} = $1; # populate parsed info from this line
...
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