exists() is a function used by hash data structures. You are calling it on array elements. Try: 
print DATA "\+$exp[1]" if $exp[1];

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That will print as long as the value in $exp[1] is not 0.
update:Response tailored to user's evident < 5.6 version of perl...see below comments for versions >= 5.6

In reply to Re: exists and defined functions by insensate
in thread exists and defined functions by arrow

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