Green’s Dictionary of Slang

1. a heavy fall.

1850		F.E. Smedley Frank Fairlegh (1878) 272: ‘Clap your leg over,’ – ‘An elephant half as high as this room [...] ram in the persuaders, and if you do get a purl’ – ‘Look upon it as the purest, brightest gem in your noble father’s coronet, for true affection’.
1856		C. Reade It Is Never Too Late to Mend II 171: They went a tremendous pace with occasional stoppages when a purl occurred.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).

2. whirling or pitching head-first or head-over-heels.

1837

Comic Almanack Apr. 87: Vell, if ever I seed such a purl as that.