I was thinking "what language is that?" before I even started reading, so yeah, I spotted it.

Now you could say that was because you removed everything but the problem, but it's just as easy to discover analytically. What does an assignment try to modify? It's LHS. What's wrong with "message"? Well, that's obvious.

Warnings don't care about bare words, it's strict.

$ perl -ce'use strict; message = "foo";'
Can't modify constant item in scalar assignment at -e line 1, near ""f
+oo";"
Bareword "message" not allowed while "strict subs" in use at -e line 1
+.
-e had compilation errors.
[download]

Note that strict only cares if the bareword is not allowed by some other rule, such as if it appears to be a sub call.

$ perl -ce'use strict; sub message; message = "foo";'
Can't modify non-lvalue subroutine call in scalar assignment at -e lin
+e 1, near ""foo";"
-e had compilation errors.
[download]

The above code is parsed as message() = "foo";, which is valid syntax, and it's even valid semantically if message is an lvalue sub.

$ perl -ce'use strict; sub message :lvalue; message = "foo";'
-e syntax OK
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By the way, where's the my?

$ perl -ce'use strict; my message = "foo";'
No such class message at -e line 1, near "; my message"
syntax error at -e line 1, near "my message ="
-e had compilation errors.
[download]