Green’s Dictionary of Slang

percher n.

1. a dying person [perch v.].

[UK]Bolingbroke Letter to J. Grahme 21 Jan. in Bagot Col. J. Grahme (1886) 28: The Queen is well, though the Whigs giue out that she is, what they wish her, ‘a percher’ [OED].

2. UK Und. uses [the victim is ‘perched’ in innocent vulnerability].

(a) a gullible victim for a swindle or con-game.

[US]‘Bill O. Lading’ You Chirped a Chinful!! n.p.: Perch: Sucker.
[UK]D. Powis Signs of Crime 196: Percher [...] an easy victim.

(b) a simple arrest [the victim is ‘perched’ in innocent vulnerability].

[UK]D. Powis Signs of Crime 196: Percher An easy arrest.
[UK]J. Morton Lowspeak.