#!/usr/bin/env perl6

use v6;

sub swap(Array @xs is copy, $pos = 1) {
  return @xs if +@xs <= 1;
  @xs[$pos .. @xs.end].unshift(reverse @xs[0..$pos]);
}

my @given = 1,3,5 ... 21;
my @swapped = swap(@given, 2);

say "BEFORE :" ~ ~@given;
say "AFTER  :" ~ ~@swapped;

[rml@glamdring mathoms]$ ./varfun.pl
Nominal type check failed for parameter '@xs'; expected Positional[Array] but got Array instead
  in 'swap' at line 5:./varfun.pl
  in main program body at line 11:./varfun.pl