#!/usr/bin/perl

package Unpack::Format;

use warnings;
use strict;
use Carp;
use overload 
    '""'  => \&templatestr,
    '@{}' => \&fieldnames;

sub new {
    my $class = shift;
    my $self = {};
    bless $self, $class;
    @_ % 2 and croak "Odd number of parameters passed to ".__PACKAGE__."::new!";
    my %params = @_;
    $self->_load_fields($params{spec});
    return $self;
}

sub _load_fields {
    my $self = shift;
    my $spec = shift;
    @$spec % 2 and croak "Incorrect format specification: odd number of elements"; 
    $self->{fieldnames} = [@{$spec}[grep { !($_ %2) } 0..$#{$spec}]];
    $self->{template_ary} = [@{$spec}[grep { $_ %2 } 0..$#{$spec}]];
}

sub fieldnames { $_[0]->{fieldnames} }
sub templatestr {
    my $self = shift;
    return join " ", @{$self->{template_ary}};
}

1;

package main;
use warnings;
use strict;

my $format = Unpack::Format->new(
   spec => [
   status => 'n  ',   #   0
   time   => 'n  ',   #   2
   date   => 'N  ',   #   4
   code   => 'a16',   #   8   key
   msid   => 'a10',   #  24   key
   ]
);

my $rec = "\x{00}\x{7B}\x{06}\x{B3}\x{01}\x{32}\x{1A}\x{83}"
         .'code_value------msid_value';

my %test_vals;
print "$format\n";

@test_vals{ @$format } = unpack $format, $rec;

foreach (@$format) {
   printf " %6s => %s\n",$_, $test_vals{$_};
}