Are these grouping parens or list creation parens?

There's no such thing as list-creating parens. Lists are created by commas and where they are needed. Parens override precedence, modify certain operators (e.g. = vs ()=, x vs ()x, eof vs eof(), etc), and indicate a lack of operands when empty.

I'd like to know if perl is able to avoid creating a one-element list

No.

$x = ("a","b","c");
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compiles to

pushmark
const "c"
list
$x
sassign
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and not

const "c"
$x
sassign
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That case of a list in scalar or void context could be optimised (if it's known at compile-time), but the savings would be negligible since pushmark and list are extremely cheap operations O(1) operations.

Is this assignment: my $check = ($y eq $z); any less efficient than this? my $check = $y eq $z;

No. Their compilation produces identical outputs.