Using the advice I've now simplified:

You've still got the same look-behind typos as described in Re: Looking for a cleaner regex

Guessing at the meaning the look behinds, the list of words is now

.ron
steis
ste
mos
es
o
ó
í
ué
(?<!u)é
(?<!st)e
(?<!ro)n
(?<!.is)s
(?<!mos)s
(?<!eis)is
(?<!ste)is
[download]

Eliminating the obviously needless look-behinds , leaving the goofy guesses and guessing again

.ron
steis
ste
mos
es
o
ó
í
ué
é
e
n
(?<!.is)s  ## maybe meant (?<!.i)s
(?<!mos)s  ## maybe meant (?<!mo)s 
(?<!eis)is ## maybe meant (?<!e)is
(?<!ste)is ## maybe meant (?<!ste)is
[download]

If my new guess is correct the last 4 needless look behinds can be replaced by "s"

steis
.ron
ste
mos
es
ué
o
ó
í
é
e
n
s
[download]

And the single character suffix can be replaced by a char class

steis
.ron
ste
mos
es
ué
[oóíéens]
[download]

So there you have it the optimal regex is  qr/( .ron | steis | ste | mos | es | ué | [oóíéens] )$/x