Oops, thanks for pointing this out. It looks like I approached the problem from the wrong side - instead of trying to prevent socketpair() from putting anything in $! I should have reset $! when checking its value is the only mean to check for errors.

So, it's more accurate to use the following:

$! = undef;
$) = $run_gid;
die "Setting egid: $!" if $!;
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(e.g., put the assignment close to when you do the call which may fail)