A bit more information: the errors always seem to happen when I make a method call (pump from IPC::Run, or some of our internal module methods). The data structures used in the vicinity of where I'm seeing frequent errors are extremely, um, baroque. A few representative lines:

${$children->[0]->{'harness'}}->{'next'} = $inUse;
$children->[0]->{'harness'} = $inUse;
$inUse = $$inUse->{'next'};
${$children->[0]->{'harness'}}->{'next'} = undef;
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Please note that I didn't write this code, and would rather rewrite it than debug it, but that may not be an option right now.