sub empty {}
if ( empty() ) { ... } # ok
@foo = empty();
if ( @foo ) { ... } # ok

sub returns_undef { undef }
if ( returns_undef() ) { ... } # ok
@foo = returns_undef();
if ( @foo ) { ... } # NOT OK

if ( grep { defined } @foo ) { ... } # oh ok, now it's "better".