Pearl undergoes a change when she perceives the difference wrought in her mother after Hester has taken down her hair and removed the scarlet "A." Hester has been speaking with the Reverend Dimmesdale in the forest, and when Pearl returns, Pearl's demeanor changes because "since [she had] rambled from her [mother's] side, another inmate had been admitted within the circle of the mother's feelings, and so modified the aspect of them all" and "Pearl, the returning wanderer, could not find her wonted place, and hardly knew where she was." Seeing her mother so close to another person—when it has always been only Pearl and Hester against the world—shook Pearl to her core. On top of this, she's only ever known her mother to wear her hair hidden under the cap and to wear the scarlet letter; now, with her hair down and the letter gone, Pearl finds her mother to be frightening and foreign.  

As a result, Pearl will not approach her mother and remains on the other side of the brook. She will not speak but only points at "her mother's breast," frowning and "stamp[ing] her foot with a yet more imperious look and gesture." Despite her mother's entreaties that she come and speak to the minister, despite even her threats of anger, Pearl

burst into a fit of passion, gesticulating violently, and throwing her small figure into the most extravagant contortions. She accompanied this wild outbreak with piercing shrieks . . .

This is a huge departure from Pearl's typical behavior. She always tends to stick with her mother no matter what else is going on, even defending her against the children in the town. Once Hester puts her hair back up and repins the letter to her breast, Pearl accepts her and returns to her old self once again.

The exact moment of Pearl's transformation in Nathaniel Hawthorne'sThe Scarlet Letteroccurs in chapter XXIII, after Arthur Dimmesdale gives his final election day speech addressed to the "people of New England", and where he confesses to be the sinner upon which the scarlet letter should have been bestowed.

It all begins when, after the final disclosure of his sin, Arthur Dimmesdale asks Pearl for a kiss. The first time Dimmesdale had kissed Pearl, it was when he met Hester in the forest; a forest whose darkness was symbolic of the secrecy and sin that still permeated the relationship between he and Hester. For this reason, Pearl rejected him then, and washed his kiss off her cheek.

This time, as the narrator explains, was different. Something supernatural seems to have taken place:

Pearl kissed his lips. A spell was broken. The great scene of grief, in which the wild infant bore a part, had developed all her sympathies ... Towards her mother, too, Pearl's errand as a messenger of anguish was all fulfilled.

We know that shortly after Dimmesdale's death, comes Chillingworth's own death. As an act of perhaps guilt, or redemption, Chillingworth leaves his possessions to Pearl, of all people. As a result of that, Pearl is able to take care of her mother and the full circle of Pearl's life is complete.