A quick fix might be to use this: 
if ($Do_Dir eq '') {

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But, if you're interested in why your code doesn't work... Tip #6 from the Basic debugging checklist, B::Deparse 

if ("$Do_Dir" == 'null') {

[download]

warnings also generates a message to let you know there is a problem.

In reply to Re: Check For null Arguments by toolic
in thread Check For null Arguments by rfleisch

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