open my $fh, '+<:utf8', 'file.in.xml' or die $!;
my $line = <$fh>;
$line=~s/<\?xml.+encoding="\Kutf-16"/utf-8" /
	or die "didn't match line: '$line'";
seek $fh,0,0 or die $!;
print $fh $line;
close $fh;