perhaps it's the fact that you're building a structure out of our 100M file in RAM that's the bottleneck

Quite possible. Data takes much more space as Perl variables than as it does in a file. Especially since hash keys are strings. 

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");

[download]
File size:    64 bytes
Memory usage: 922 bytes

[download]

In reply to Re^2: Unpacking small chucks of data quickly by ikegami
in thread Unpacking small chucks of data quickly by Anonymous Monk

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