print "%CALC{\$EVAL(R${c}:C2 - R${pi}:C2)}%";
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Perl can't divine where your $c and $pi variable names start/end in the string, so you have to wrap them in curly braces.

Edit: Whoops, the code is right (I think), but the reasoning is wrong. Didn't notice that a colon immediately followed the variable names; that renders what I said before moot.

Edit2: Ah, toolic has pointed out the real problem already.