Read description of $? carefully. There it states that if SIGCHLD is set, then $? shall be incorrect. Check the following code

system('true');
print "\$? is $?\n";

$SIG{CHLD} = 'IGNORE';

system('true');
print "\$? is $?\n";
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From this follows, that most likely your DBI call has set the $SIG{CHLD} and didn't clear it.