Now, brethren and sistern, let's all charitabley agree to stipulate that the horrid "map in a void context" was merely for the purpose of reducing the problem to its bare essentials.

It would appear that, because of what you have put inside the {...}, Perl is taking that to be an expression and thus needing a comma after it.

use strict;
my @l = 1..10;
map { 1, 2 }, @l;
map { "$_", 2 }, @l;
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does not produce errors. And if you say:

print map { 1, 2 }, @l;
print map { "$_", 2 }, @l;
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you will see a bunch of hash-refs. So Perl is seeing {1,2} as an anonymous hash rather than as a BLOCK that happens to return something. (A very reasonable reading now that we look at it that way.)

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"Perl is a mess and that's good because the
problem space is also a mess." - Larry Wall