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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pushmark
const "c"
list
$x
sassign
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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.

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.

That's very enlightening - thanks!

>perl -MO=Deparse -e "my $check = ($y eq $z);my $check = $y eq $z;"
my $check = $y eq $z;
my $check = $y eq $z;
-e syntax OK
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Also, if Perl really were creating a one-element list, that one-element list would be true even if it only contained one undef element:

> perl -wle "my @hypothetical = (1 == 0); my $check = @hypothetical; p
+rint $check"
1
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So, Perl knows an expression when it sees one, and the expression is a simple boolean result, and nothing complicated.

Mostly though, if you are worrying about such minuscule optimizations, you should either be using C or spend the time about rethinking your approach to the problem, possibly either reworking your algorithms or reworking your data structures. In both, you'll find far more possibilities to speed up your program than in musings about how Perl evaluates boolean expressions.

Even I, who takes pessimisation avoidance seriously, is (am?) having considerable trouble taking this question seriously. (Geez that is clumsy wording.)

More so because you purport to be worried enough to ask the question, but not enough to take the simplest steps to discover the answer for yourself. Vis:

C:\test>perl -MO=Deparse -e" my $check = ($y eq $z); "
my $check = $y eq $z;
-e syntax OK

C:\test>perl -MO=Deparse -e"my $check = $y eq $z;"
my $check = $y eq $z;
-e syntax OK
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Why bother responding. Simple. Cos it took less tham 2 minutes to do so--and the adverts last 3.