use List::Util qw(max);
@last_n = @source[max(0, @source - $n) .. $#source];
[download]

Update: Fixed offby1-error pointed out by AnomalousMonk++

See What's the best way to get the last N elements of a Perl array? on stackoverflow.com.

@last_n = grep { defined($_) } @last_n;
[download]

Seems like you already tried something or at least thought about your task. Was there any outcome (a.k.a. code) that you can show us?

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.

nope, does not work for me: Modification of non-creatable array value attempted, subscript -9 at array_ende.pl line 23. whereas it does work as a two-liner (see above)
Regards,
svenXY

Beware: What does that solution do, if $n > @source?

See:

$ perl -le '@a = 1..3; @b = @a[(@a-5)..$#a]; print "<$_>" for @b;'
<2>
<3>
<1>
<2>
<3>
[download]

@last_n=@source[(@source-$n)..$#source]if ($n<=@scource);
[download]

Would be the solution there I guess