What you might consider doing is making sure that you aren't forking with any database handles open. Shut them all down, fork, and bring them back as required. For example:

my $dbh = DBI->connect(...);

$dbh->do(...);

$dbh->disconnect();

foreach (...) { ... fork ... }

$dbh = DBI->connect(...);

$dbh->do(...);

$dbh->disconnect();
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This would look a lot better if you organized things into functions.

I think what is occuring is that the forked scripts are trying to close out the DBI connection that they inherit from the parent process. Since you can't close them more than once, you're in trouble just having them around at all when forking.