[oak perl] The mysterious "undef"

[Sandy Santra]

I keep seeing "undef" mentioned in the llama book.

I've tried to define a variable as not having a value, theoretically being 
"undef," but when I use the print command, I never see "undef" print to my 
screen; nothing prints at all.

I wanted to learn the different between a zero value and undef because the 
llama book keeps saying there is one.  Is this a pointless issue to pursue?

Clarification greatly appreciated.  Thanks.