use strict;
use warnings;

use Devel::Size qw( total_size );

my $file = join '', map pack('NN', @$_), (
  [ 273, 1234 ],
  [ 273, 5678 ],
  [ 274, 1234 ],
  [ 275, 5678 ],
  [ 276, 1234 ],
  [ 277, 5678 ],
  [ 278, 1234 ],
  [ 278, 5678 ],
);

my %ValArrayByID;
while ($file =~ /(.{8})/g) {
   my ($ID, $Val) = unpack('NN', $1);
   push @{$ValArrayByID{$ID}}, $Val;
}

print("File size:    ", length($file),              " bytes\n");
print("Memory usage: ", total_size(\%ValArrayByID), " bytes\n");

##</code><code>##

File size:    64 bytes
Memory usage: 922 bytes