Hi,
My problem/confusion can be illustrated with the code below: 
#!/usr/bin/perl
use strict;
use threads;
use threads::shared;

my %h:shared;
for (0..10) {
        $h{$_}=&share({});
}
for (keys(%h)) {
        print "$h{$_}\n";
}

[download]
On my system, I get: 
HASH(0x9904d20)
HASH(0x9904d38)
HASH(0x9904d14)
HASH(0x9904d2c)
HASH(0x9904d08)
HASH(0x9904d20)
HASH(0x9904d38)
HASH(0x9904d14)
HASH(0x9904d2c)
HASH(0x9904d08)
HASH(0x9904d20)

[download]
Now, obviously, the addresses change with each invocation, but it is always 5 unique addresses that repeat. It seems to be creating 11 distinct hashes where I can file away my scalars, but it doesn't make sense to me that there would only be 5 unique memory addresses. Now, mysteries like this always bother me, but beyond the mystery, this behavior breaks XML::Dumper as it uses the memory address to ensure that a data structure that is referenced multiple times is only dumped once. Without the threads::shared module, I get 11 distinct memory locations as I would expect as illustrated below: 
#!/usr/bin/perl
use strict;

my %h;
for (0..10) {
        $h{$_}={};
}
for (keys(%h)) {
        print "$h{$_}\n";
}

[download]
Output: 
HASH(0x9ea67c8)
HASH(0x9ea6720)
HASH(0x9ea67b0)
HASH(0x9ec8578)
HASH(0x9ea66e4)
HASH(0x9ea672c)
HASH(0x9ea5b44)
HASH(0x9ea69a8)
HASH(0x9ea6600)
HASH(0x9ec8590)
HASH(0x9ea6990)

[download]
What am I missing? Is there a good reason for this behavior? It has rendered XML::Dumper useless for me.
Eric

In reply to mysterious threads::shared behaviour by ericatkin

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