Ok today I ran into an unexpected behavior. I have a script which generates an input string for a JavaScript menu/submenu thing. The function that generates the string builds this string based on items in the database. However if the form was just submitted I want the current categories to stick ala CGI.pm's default behavior. So I pass the current value of those params as the first two arguments to my subroutine. I have an actual third argument which if true adds an All option to each of the menus. My problem was on the initial load I wasn't getting the all menu which led me to discover a surprising behavior in Perl I wasn't aware of. 
sub makeMenuInput{
   my ($category,$subcategory,$allIsOn)=@_;
}

makeMenuInput($query->param('category'),$query->param('subcategory'),"
+allon");

[download]
The surprising thing that happened when 'category' and 'subcategory' weren't defined was that the "allon" option got passed in the first slot.

What I ended up having to do to get around this was: 

makeMenuInput($query->param('category')||"",$query->param('subcategory
+')||"","allon");

[download]
Can anyone explain why this occurs?

In reply to Unexpected behavior after passing $query-param('bar') as subroutine parameter by Banky

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