Since 'a' is true, ('a' or ...anything...) is the same as 'a'. Therefore,
when ('a' or 'b') is the same as when ('a')
when ('a' or $_ ~~ 'b') is the same as when ('a')

The third one is wrong too. It'll say yes no matter what $x contains since ($_ ~~ 'a' or 'b') is always true.

test('c')
[download]

gives

N
N
Y
[download]