use feature "switch";

for (to be determined) {

when ((s/(\b[A-E]{3}\b)/XXX/g)) {@array[i,1] = $1}
when (s/((Science)|(science))/XXX/g) {@array[i,2] = $1}
when (s/((Math)|(math))/XXX/g) {@array[i,3] = $1} 
when - for all remaining subjects...
when (s/((first|First)\W+(?:\w+\W+){1,10}?([A-E]{3})) #or some similar REGEX returning only the first year entry grades
default {}
}