That's not a list, that's the comma operator in scalar context, which will disregard its left hand value and return its right hand value.
So in my $x = ('a', 'b', 'c');, 'a' and 'b' get discarded, and 'c' gets assigned to $x, as your snippet proves. This behaviour is documented (see the link I gave) and also demonstrated by B::Deparse:
$ perl -MO=Deparse -E '$x = ("a", "b", "c");'
use feature 'current_sub', 'evalbytes', 'fc', 'say', 'state', 'switch'
+, 'unicode_strings', 'unicode_eval';
$x = ('???', '???', 'c');
-e syntax OK
However, =()= is an idiomatic way to enforce list context. The details are better explained in Perl Idioms Explained - my $count = () = /.../g than I could do it myself, but just to demonstrate that it works, consider the following.
$ perl -MO=Deparse -E '$x =()= ("a", "b", "c"); print $x'
use feature 'current_sub', 'evalbytes', 'fc', 'say', 'state', 'switch'
+, 'unicode_strings', 'unicode_eval';
$x = () = ('a', 'b', 'c');
print $x;
-e syntax OK
$ perl -E '$x =()= ("a", "b", "c"); print $x'
3
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