I just hate an unfindable person!
(Well, I love one, actually -- but to a point.

)
Do you suppose she may just have hated her name? (I know what my nephew's response was when he was about 15 and learned he had a greatx3 grandmother named Dorcas. I'm not sure which he liked better, that or his greatx2 grandmother Alice Cooper.)
1841 in Netheravon, Charlton, Wiltshire:
John Perry, 55, ag lab
Tabitha Perry, 60 (c 1777-81)
There is no Tabitha Perry death recorded to match the above TP.
Especially if I was right that Dorcas Turner was a widow when she married John Perrior, the age difference could fit, as well. And there'd be nothing to stop a gal who was older than her husband from minimizing the difference a bit.
John Perry also seems to have died by 1851.
The name of the informant on Dorcas Perry's death certificate could help considerably -- if it was her husband, in particular. You might also want to submit a correction to FreeBMD, since her death entry is now shown as "Dorca_" and it took me a while to figure out why I couldn't find it.

In the 1841 census, there are three young Perrys on the facing page:
Elizabeth 18
John 15
Frederick 14
and one might think they were related, if that could help. Yes, as suspected -- the household is headed by a Blackman couple:
John 35, ag lab
Mary 45
and she was Mary Perry; married in 1831 so the children may have been hers from a previous marriage, argh:
https://familysearch.org/pal:/MM9.1.1/NKQY-R8V