Is determiner 'a' needed in "one would call such a value a constant"?

It's grammatical to say "a constant" in the first sentence. The indefinite article "a" is required because "constant" is a count noun.

In your second example, you're right, that is also grammatical. HOWEVER, "constant" is appearing as an adjective in that sentence, not as a noun. If you want to use "constant" as a noun, you need to put "a" beforehand in that sentence.

Both of your example sentences are grammatically correct, but they mean subtly different things.

"One would call such a value a constant" means that "a constant" is another name for a value with the characteristics referred to by "such".

"One would call such a value constant" means that "constant" is an adjective describing the quality possessed by values with the characteristics referred to by "such".

In this case, these two sentences say very nearly the same thing, because English often allows you to say "a [adjective]" to mean "an object with the quality [adjective]". However, this doesn't always work: "X is a constant" is fine but "*X is a red" demands the reply "A red what?" And, for the same reason, you can only say "One would call such a value red", not "*One would call such a value a red".

(Phrases marked with a * at the beginning are things that native speakers would never say.)