@a = ("a", 1, "b", 2, "c", 3);
$i = 0;
grep($foo{$_} = $a[$#a - $i++], @a);

# for each element $_ in @a
# set $foo{$_} to the element OPPOSITE $_ in the array
# %foo = (
#   a => 3, 1 => 'c', b => 2,
#   2 => 'b', c => 1, 3 => 'a',
# )

print $foo{"c"}, "\n";  # 1