Actually, to my surprise, simply sticking an ID number in the refered-to object (it's an array) and dereferencing it is fastest. Here are the numbers: 
Benchmark: running ID Field, Numeric, Refaddr, String for at least 3 C
+PU seconds...
  ID Field:  4 wallclock secs ( 3.43 usr +  0.03 sys =  3.46 CPU) @ 10
+44124.57/s (n=3612671)
   Numeric:  4 wallclock secs ( 3.34 usr +  0.04 sys =  3.38 CPU) @ 98
+9664.50/s (n=3345066)
   Refaddr:  5 wallclock secs ( 3.08 usr +  0.03 sys =  3.11 CPU) @ 91
+2330.23/s (n=2837347)
    String:  4 wallclock secs ( 3.09 usr +  0.04 sys =  3.13 CPU) @ 74
+7332.27/s (n=2339150)

[download]
              Rate   String  Refaddr  Numeric ID Field
String    747332/s       --     -18%     -24%     -28%
Refaddr   912330/s      22%       --      -8%     -13%
Numeric   989664/s      32%       8%       --      -5%
ID Field 1044125/s      40%      14%       6%       --

[download]
And here's the code that did the Benchmark:
#!perl -w

use strict;
use Benchmark qw(:all);
use Scalar::Util qw(refaddr);

my $r = [0];

sub string {
    pack("A", "$r");
}

sub numeric {
    pack("J", $r+0);
}

sub addr {
    pack("J", refaddr $r);
}

sub field {
    pack("J", $r->[0]);
}

my $result = timethese(-3, {
    String       => \&string,
    Numeric      => \&numeric,
    Refaddr      => \&addr,
    "ID Field"   => \&field,
});

cmpthese($result);

[download]

In reply to A benchmark by Jeffrey Kegler
in thread Unique numeric ID for reference? by Jeffrey Kegler

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