sub fillDataFromHash
{
  my ($src, $recordSpec, $data) = @_;
  $data ||= ' ' x $RECORD_LENGTH;  # Maybe add to %recordformat
  my @spec = @$recordSpec;
  while (@spec)
  {
    my $key = shift @spec;
    my $location = shift @spec;
    $location =~ /(\d+)(?:-(\d))?/ or
        die "Syntax error in recordSpec: @$recordSpec";
    my ($start, $end) = ($1, $2 || $1);
    substr($data, $start - 1, $end - $start + 1) = $src->{$key};
  }
}

# ...
# fill %datavalues somehow
my $dataLine = fillDataFromHash(\%datavalues, 
                                $recordformat{__COMMON__});
$dataLine = fillDataFromHash(\%datavalues, 
                             $recordformat{$datavalues{'RecordType'}}, $dataLine);