Hi Emmanuel,
I cant quite explain this fully, but if I copy-and-paste your string below
and print it in hex, then:
eedjsa@server68:~/apl-1.8/src$ echo 'Z ← L foo R' | od -t x1z
0000000 5a 20 e2 86 90 20 4c 20 66 6f 6f 20 52 0a >Z ... L foo R.<
0000016
which suggests that the sequence
e2 86 90 (which, by chance, is
the UTF8 encoding of U+2190 aka. ←) made it into the tokenizer
of GNU APL (which then correctly complains).
In other words, it very much looks to me as if some conversion from
UTF8 encoding to Unicode (aka. UCS) is missing on the way of your
header line from emacs to APL.