$ perl -MList::Util=sum -Wle 'printf "sum returns [%s]", sum()'
Use of uninitialized value in printf at -e line 1.
sum returns []

$ perl -MList::Util=sum -Wle 'printf "sum returns [%s]", sum(undef)
Use of uninitialized value in subroutine entry at -e line 1.
sum returns [0]