Depending on your data (e.g. if it's ok to null out a 0), you could simply do: 
$case->owner(  $owner || undef  );

[download]
This is great writeup on what counts as false: True or False? A Quick Reference Guide

In reply to Re^2: class-dbi-mysql and null values by davidrw
in thread class-dbi-mysql and null values by davidj

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