our (@fields, %width, $packt);
{
  my @f = (
    # list of field names and widths in the order in which they appear
    li   => 2, 
    vn   => 3,
    mode => 3,
    (map {$_ => 8}  qw(stratum poll_interval precision)),
    (map {$_ => 32} qw(root_delay dispersion identifier)), 
    (map {$_ => 64} qw(ref_epoch originate_epoch receive_epoch transmit_epoch))
  );

  @fields = map {$f[$_*2]} (0 .. @f/2);
  %width = (@f);
  $packt = join '', map {"a$width{$_}"} @fields;
}

...

%srv = ();
@srv{@fields} = map { oct("0b".$_) } unpack $packt, $received_data;