If undef is no valid entry in your array, you could use your solution and grep the defined values from your slice result.

@last_n = grep { defined $_ } @source[ -$n .. -1 ];
[download]

Update:

As svenXY mentioned: my solution does not work! :o(

$ perl -wle '@a = 1..3; @b = grep { defined $_ } @a[-4 .. -1]; print "
+<$_>" for @b;'
Modification of non-creatable array value attempted, subscript -4 at -
+e line 1.

$ perl -wle '@a = 1..3; @b = @a[-4 .. -1]; @b = grep {defined $_} @b; 
+print "<$_>" for @b;'
<1>
<2>
<3>
[download]

Can someone please enlighten me, what's the problem with my attempt?

2nd Update:

During my research I found an explanation in Modification of non-creatable array value attempted (it's a slightly other topic, but at least the same error message) and grep's perldoc.

I came to the conclusion, that the error is thrown, because grep tries to alias the list values; and this fails for not existent array elements.