• the return list is constructed at compile time.
• at runtime, the list (@x, @y) is evaluated in the caller's context, so it resolves to the comma operator, returning it's last argument - @y
• then the array @y is evaluated in the caller's context, which happens to be scalar, hence 4 as a result.

Pretty much the same as

perl -le '@x=qw(a b c);@y=qw(a b c d); $f=(@x,@y); print $f'
4
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;-)

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}