Close, but you're missing a set of parens
my ($mac, $ip) = (split)[3,5];
However, I prefer to be more explicit and not depend on the 2 separate defaults ($_ and the split pattern) which a new Perl programmer may not know about.
So, in production code, I'd be specifying the pattern and use a named var instead of $_.
So, I'd normally do this:
my ($mac, $ip) = (split(/ /, $line))[3,5];
I've seen people do this:
which has the added problem of not knowing what kind of data you're processing.