Excel allows cross sheet references with underscores without having to escape
the sheet name: e.g: =Cash_Flow!A1
POI's FormulaParser however cannot handle such references because an underscore
is not a character letter and isAlpha(char c) returns false.

Solution could be to change the method isAlpha(char c) in
org.apache.poi.hssf.model.FormulaParser from

return Character.isLetter(c) || c == '$';

to

return Character.isLetter(c) || c == '$' || c == '_';