Here's my take on Q1, which isn't entirely original, but is self-contained:

my @c = do { my %tmp = map { $_ => 1 } @a; grep {$tmp{$_}} @b };

Here's my take on Q2 which ended up the same as others:

for (1..@foo/2) { @foo[$_-1,-$_] = @foo[-$_,$_-1]; }

The first thing that came to mind was:

@foo = map { pop(@foo) } 1..@foo;

But that's not precisely an in-place version, even though you could argue that the temporary "array" only holds things that have been removed from @foo.