#!/usr/local/bin/perl
use 5.012;

sub mapBetween(&@) {
    map { $_[0]->( $_[$_], $_[ $_ + 1 ] ) } 1 .. @_ - 2;
}

say '# ', join ",", mapBetween { $_[1] - $_[0] } ();
say '# ', join ",", mapBetween { $_[1] - $_[0] } (0);
say '# ', join ",", mapBetween { $_[1] - $_[0] } (0, 1);
say '# ', join ",", mapBetween { $_[1] - $_[0] } (0, 1, 4);
say '# ', join ",", mapBetween { $_[1] - $_[0] } (0, 1, 4, 9);