I've seen quite often that Perl was reusing reference addresses after garbage collection had them released before.

I was curious as to how often this address reusing happens:

#!/bin/bash

for i in {1..15}; do
perl -Mstrict -wle'
use strict;
use warnings;

my %addr;
my $count = 0;
my $tries = 2**$ARGV[0];
for my $try (1..$tries) {
    my %h;
    for (reverse 1..1000) {
        $h{$_} = { value => $_};
        $count++;
    }

    $addr{$_}++ for values %h;
}

my $reused = scalar grep $addr{$_}>1, keys %addr;
my $addrs  = scalar keys %addr;
print "Address of reference reused $reused times, all addresses $addrs
+, in $count tries";' $i
done
[download]

Address of reference reused 1 times, all addresses 1999, in 2000 tries
Address of reference reused 1001 times, all addresses 2999, in 4000 tr
+ies
Address of reference reused 2995 times, all addresses 3007, in 8000 tr
+ies
Address of reference reused 3011 times, all addresses 3023, in 16000 t
+ries
Address of reference reused 3043 times, all addresses 3055, in 32000 t
+ries
Address of reference reused 3107 times, all addresses 3119, in 64000 t
+ries
Address of reference reused 3235 times, all addresses 3247, in 128000 
+tries
Address of reference reused 3491 times, all addresses 3503, in 256000 
+tries
Address of reference reused 4003 times, all addresses 4015, in 512000 
+tries
Address of reference reused 5027 times, all addresses 5039, in 1024000
+ tries
Address of reference reused 7075 times, all addresses 7087, in 2048000
+ tries
Address of reference reused 11171 times, all addresses 11183, in 40960
+00 tries
Address of reference reused 19363 times, all addresses 19375, in 81920
+00 tries
Address of reference reused 35747 times, all addresses 35759, in 16384
+000 tries
Address of reference reused 68515 times, all addresses 68527, in 32768
+000 tries
[download]

These numbers seem to be stable on my machine across multiple runs.

The bottom line is that address reusing happens all the time.