sub _device_read{
   my $d = shift;  # reduced line to relevant parts

   ...

   # parse device config
   $d->{regs}=from_json( $d->{regs} );

##</code><code>##

sub _device_read{
  ...
  $r->{$reg} = _prittyfy_byte(...);
  ...
};

sub _prettify_byte{
    return (...) if( $_[0] );
    return( $c{_err_} );
};

##</code><code>##

my %registers = %{$d->{regs}->{r}};
foreach my $reg (keys %registers){
  my $reg_id  = hex $reg;
  my $reg_num = $registers{$reg};

  # read_input_registers returns an array reference
  my $tmp = $mbc->read_input_registers($reg_id, $reg_num);

  $r->{$reg} = _prettify_byte($tmp);
};

##</code><code>##

my %registers = %{$d->{regs}->{r}};
foreach my $reg (keys %registers){
  # read_input_registers returns an array reference
  my $tmp = $mbc->read_input_registers(hex $reg, $registers{$reg});

  $r->{$reg} = _prettify_byte($tmp);
};