Wordplay here involves sorel, a deer of the third year;
sore, a deer of the fourth year; L, the Roman numeral fifty

... and of course also much scabrous wordplay on defloration of a virgin (pierc'd and prick'd a pretty pleasing pricket...Some say a sore, but not a sore, till now made sore with shooting which uses the allusion to the difference between a 3 and 4 year old deer as a metaphor for sexual knowledge. Good ol Will!