when you fork everybody gets their own independent/unrelated/unshared copy of $pwm_count_hash . As each process updates $pwm_count_hash, the others don't see these updates, variables aren't shared like that

See Re: Please Explain the Parallel::ForkManager Idiom my $pid = $pm->start and next;, Mr. Peabody Explains fork()

In reply to Re: value uninitilized when using Parallel::ForkManager module by Anonymous Monk
in thread value uninitilized when using Parallel::ForkManager module by plagent

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