perl -wMstrict -le
"sub standard_commas {
  return @_ == 0 ? ''
       : @_ == 1 ? qq{$_[0]}
       : join q{, }, @_[0 .. $#_ - 2], qq{$_[-2] and $_[-1]}
  }
 sub oxford_commas {
  return @_ < 3 ? standard_commas(@_)
                : join q{, }, @_[0 .. $#_ - 1], qq{and $_[-1]}
  }
 for (@ARGV) {
  my @words = split;
  print qq(\nfor words: @words);
  print 'standard commas: ', standard_commas(@words);
  print '  oxford commas: ',   oxford_commas(@words);
  }
"
""   "a"   "a b"   "a b c"   "a b c d"   "a b c d e"

for words:
standard commas:
  oxford commas:

for words: a
standard commas: a
  oxford commas: a

for words: a b
standard commas: a and b
  oxford commas: a and b

for words: a b c
standard commas: a, b and c
  oxford commas: a, b, and c

for words: a b c d
standard commas: a, b, c and d
  oxford commas: a, b, c, and d

for words: a b c d e
standard commas: a, b, c, d and e
  oxford commas: a, b, c, d, and e
[download]

And, since I don't seem to be able to let this go, still shorter and sweeter:

sub standard_commas {
    return join q{, },
        @_ < 2 ? @_
               : (@_[0 .. $#_ - 2], qq{$_[-2] and $_[-1]})
    }

sub oxford_commas {
    return join q{, },
        @_ < 3 ? goto &standard_commas
               : (@_[0 .. $#_ - 1], qq{and $_[-1]})
    }
[download]

Tested. The output, which I won't inflict upon you again, is the same.