#! perl -sw
use strict;
sub npAndAList{$"=', '; return unless @_; @_==1 and "@_" or "@_[0 .. $#_-1] and $_[-1]" }
print npAndAList(qw(a b c d e)[0..$_]),$/ for -1..4;
__DATA__
a
a and b
a, b and c
a, b, c and d
a, b, c, d and e