sub tst { return undef } means returning a one element list in list context.

so @a = tst() is true in if(@a) {..} (length is one)

FWIW for me a blank return; is best for false. (i.e. returning an empty list implicitely )

And the empty list evaluates to undef again in scalar context.

update

I've been a bit sloppy in my wording, Perl doesn't return a list!

The return statement is evaluated in the call context (one of the weirdnesses which were adressed in Perl6).

But this is not relevant when retuning just one undef.