it is surprising that it parses at all, since I thought "my" took a list of variables

I suspect that it takes any expression, then checks if the expression is valid.

>perl -e"my ( f($x, $y.) );"
syntax error at -e line 1, near ".) "
Execution of -e aborted due to compilation errors.
[download]

The catch here is that that constant folding happens before the check.

Another allowed expression:

>perl -e"my(my(my(my $sharona)))"

>
[download]

I suspect that it takes any expression, then checks if the expression is valid.

Yeah, but even then, it will reject most expressions:

$perl -we 'my(f($x))'
Can't declare subroutine entry in "my" at -e line 1, at EOF
Execution of -e aborted due to compilation errors.
[download]

Now, my does return an lvalue, and ?: can, but that isn't enough in itself for it to be accepted by my:

$ perl -we 'my($x = 3)'
Can't declare scalar assignment in "my" at -e line 1, at EOF
Execution of -e aborted due to compilation errors.
[download]

What the reason is that my(my $x) and my(0?$x:$y) are accept isn't clear to me.

This one is interesting:

$ perl -we 'my(local $x)'
Can't localize lexical variable $x at -e line 1.
$ perl -we 'perl -we 'local(my $x)'
Can't localize lexical variable $x at -e line 1.
$
[download]

Regardless of the nesting of local and my, we get the same error message.

Mixing state and my:

$ perl -wE 'sub f {state (my $x); say ++$x} f; f;'
1
1
$ perl -wE 'sub f {my (state $x); say ++$x} f; f;'
1
2
$ perl -wE 'sub f {state my $x; say ++$x} f; f;'
No such class my at -e line 1, near "{state my"
Execution of -e aborted due to compilation errors.
[download]

Mixing our and local:

$ perl -wE 'sub f {our local $x; say ++$x} f; f;'
No such class local at -e line 1, near "{our local"
Execution of -e aborted due to compilation errors.
$ perl -wE 'sub f {our (local $x); say ++$x} f; f;'
1
1
$ perl -wE 'sub f {local our $x; say ++$x} f; f;'
1
1
$
[download]

So we can have our (local $x), local our $x, but our local $x doesn't parse.

What the reason is that my(my $x) and my(0?$x:$y) are accept isn't clear to me.

my(0?$x:$y) gets constant-folded to valid my($y). The validity checker apparently runs after constant-folding.

$lex and my $lex are the same op. The only difference is the LVINTRO flag. The validity checker apparently doesn't check the flag.

Can't localize lexical variable $x

This is consistent with my guess at the workings, that any variable lookup in the my argument expression is taken to be a lexical declaration. local craps out on the lexical passed to it. (Don't forget, local doesn't declare variables.)