Are you sure that it is the original spelling, or just that it was used? So, could it have been originally spelled more than one way, with neither correct or wrong (like Shakespeare spelling his name several different ways in text), or purl was correct with it often being misspelled pearl (less likely)? Or was it definitely originally always 'pearl'?
I think that because pearl was already a word with other meanings, it's more likely that this was original (also purls could look like pearls) and it got changed. On the other hand, I have to admit that if purl were correct, I would expect it to be commonly misspelled as 'pearl' for the same reasons.