qw(foo bar))[$_[0]] 
would still yield 'foo' when @_ is empty (therefore, $_[0] is undef).
##</code><code>##

Enter h or `h h' for help, or `man perldebug' for more help.

h
main::(-:1):    h
  DB<1> x $ARGV[0]
0  undef
  DB<2> x 3 + $ARGV[0]
0  3
  DB<3> x 3 * $ARGV[0]
0  0