How can I use a variable instead of "F28"?

This essentially just goes back to toolic's original reply: make sure that  $sheet is a hash reference and that its content is what you expect; access the contents via  -> arrow notation, forming the hash keys as before with something like  $name = $sheet->{"${_}28"}; etc. (A simple concatenation like  $sheet->{$_ . '28'} would work just as well.)