use feature ":5.14";
use warnings FATAL => qw(all);
use strict;
use Data::Dump qw(dump pp);
use XML::Simple;

sub readUnicode($)
 {my ($f) = @_;
  open(my $F, "<:encoding(UTF-16)", $f) or die "Cannot open $f for unicode input";
  local $/ = undef;
  <$F>;
 }

sub writeUnicode($$)
 {my ($f, $s) = @_;
  open(my $F, ">:encoding(UTF-16)", $f) or die "Cannot open $f for unicode output";
  say {$F} $s;
 }

my $x = XMLin(readUnicode("some file containing XML"));
$x->{foo}{bar} = "new value";
my $X = XMLout($x);

writeUnicode("output file name", $X);