I suppose you mean "associative" array, that is a "hash" in Perl.

Instead of relying on nifty regexes to process whole records in one string I'd rather prefer using a flip-flop like demonstrated here: Re: Parsing file and joining content into string to read al tags from TAGSTART through TAGEND

in the if part do something like

   @tag = split ' ', $line, 2;
   $field{ $tag[0] } = $tag[1];
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and in the else part

   $bill{ $field{ACCOUNT} } = $field{BILLAMOUNT}  if %field;
   %field = ();
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IMHO this is a more robust and better maintainable approach.

UPDATE:

For instance if you ever need to retrieve all data per account just do

  $bill{ $field{ACCOUNT} } = { %field }  if %field;
  %field = ();
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