use strict;
use warnings;

my $Profile = "C:\\registry.reg"; # unicode file
my $old_name = 'Foo';
my $new_name = 'Bar';

my @reg_file;
{
    open(my $fh, '<:raw:perlio:encoding(UTF-16):crlf', $Profile)
        or die("Can't open file $Profile: $!\n");
    @reg_file = <$fh>
}

for (@reg_file) {
    s/
        ^
        ( \[ HKEY_LOCAL_MACHINE\\SOFTWARE\\ )
        $old_name
        ( (?:\\.*)? \] )
    /$1$new_name$2/x;
}

{
    open(my $fh, '>:raw:perlio:encoding(UTF-16le):crlf', $Profile)
        or die("Can't overwrite file $Profile: $!\n");
    print $fh "\x{FEFF}", @reg_file;
}