This works and does what I want...

nci_with_pir

sub rexp(*@args) {
  return Q:PIR {
    $P0 = find_lex '@args'
    .local num arg1
    arg1 = $P0[0]
    .local pmc libRmath, rexp
    .local num ans
    libRmath = loadlib "libRmath"
    rexp = dlfunc libRmath, "rexp", "dd"
    ans = rexp(arg1)
    %r = box ans
  };
}