threads::shared variables are *very* slow; you should share little, and access it not a lot.

The implementation essentially does a similar thing to tieing (except that it's implemented in XS rather than perl); so

my %hash : shared;
...
$x = $hash{foo};
[download]

is a bit like

sub threads::shared::FETCH {
    lock $Some:Global:lock_var;
    return $Some::Shared::Space::hash{$_[0]};
}

my %hash;
tie %hash, 'threads::shared';
...
[download]

Note that each thread has its own copy of the 'tied' hash; accessing it causes a global lock to be set, then an entry from the 'real' hash is copied to that thread.

Dave.