The "aul" is mentioned only two times in the Bible. Here are the references:
Exodus 21:6 Then his master shall bring him unto the judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him for ever.
Deuteronomy 15:17 Then thou shalt take an aul, and thrust it through his ear unto the door, and he shall be thy servant for ever. And also unto thy maidservant thou shalt do likewise.
The "aul" is simply another spelling for the word "awl." Historically, this word has been spelled in many ways (given here in general historical order): ael, eal, awul, awel, al, owul, eawl, eaule, owel, ouel, el, oule, alle, aule, ele, awle, all, aul, awl. This is a small tool with a long, sharp point used for making holes, usually in leather or in wood (to prepare the wood to take a nail). However, in the Bible examples, it was clearly used to make a hole in the ear of the Hebrew servant who wanted to give up his chance to go free and surrender as a lifelong servant to his master.
Basically, it was a sign of voluntary slavery. The servant was brought to a door or a door post and the master would place the servant's ear up against the wood and use the aul to bore a hole in his ear. We are not told whether or not the servant placed an ornament such as an earring in his pierced ear. According to the "Cyclopedia of Biblical, Theological, and Ecclesiastical Literature," it was the practice in Lydia, India, and Persia to perforate the ears of boys dedicated to the service of the gods. Also, other Oriental countries employed the piercing of the ear as a sign of perpetual servitude.
An interesting use of this typical action is seen in the following prophecy in reference to Jesus Christ:
Psalm 40:6 Sacrifice and offering thou didst not desire; mine ears hast thou opened: burnt offering and sin offering hast thou not required. 7 Then said I, Lo, I come: in the volume of the book it is written of me, 8 I delight to do thy will, O my God: yea, thy law is within my heart.
The opening of the ear in Psalm 40:6 is an allusion to the piercing of the ear of the servant as mentioned in the law. Yet, when this prophecy is quoted in the New Testament and applied to Christ, the opening of the ear takes on a new meaning.
Hebrews 10:5 Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me: 6 In burnt offerings and sacrifices for sin thou hast had no pleasure. 7 Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God.
In place of "mine ears hast thou opened" we read "a body hast thou prepared me." God is teaching us something. The opening or piercing of the ear is an act of voluntary surrender to full servitude. When Jesus came to earth and was born as a baby in the manger, thus taking on Him a body that had been prepared for Him, this was also a supreme act of surrender to servitude. He came to do the will of the Father (John 5:30). Of His incarnation (coming in the flesh), Philippians 2:7-8 states, "But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross." In His act of taking the body prepared for Him, Jesus Christ took the form of a servant. That is, He put His ears to the door post and allowed the Father to open them with His aul.