I don't know any of the following to be thread-safe: 

$shared = "abc";     # Can cause scalar type conversion
my $ref = \$shared;  # Increments ref count
print "$shared\n";   # Implicit type conversion

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Update: They are thread-safe. See Re^10: is ||= threadsafe? (threads != iThreads, "locking")

In reply to Re: is ||= threadsafe? by ikegami
in thread is ||= threadsafe? by perl-diddler

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