I would suspect that the problem resides in the way your $txt is created. I wrote up this piece of demo, to show different ways to form your string, and "use bytes" works all the time.

Hope this helps:

use strict;

sub display {
    my $string = shift;
    use utf8;# as you can see from the result, whether to use utf8, or
+ bytes is irrelevant in this demo, as "U*' forces unicode any way 
    print  "\nchar semantics: ";
    print "$string ";
    printf "Length = %d, ", length($string);
    printf "Content = %vd\n", $string;
    use bytes;
    print  "byte semantics: ";
    print "$string ";
    printf "Length = %d, ", length($string);
    printf "Content = %vd\n", $string;
}

my $encoded_string;
my @decoded_list;

{
    use bytes; 
    print "=========================\n";
    print "Case 1: create string from pack, with use bytes\n";
    $encoded_string = pack("U*", 400, 306);
    display $encoded_string;
    @decoded_list = unpack("U*", $encoded_string);
    print join(".", @decoded_list), "\n";
}

{
    use utf8; #not necessary in this case
    print "=========================\n";
    print "Case 2: create string from pack, with use utf8\n";
    $encoded_string = pack("U*", 400, 306);
    display $encoded_string;
    @decoded_list = unpack("U*", $encoded_string);
    print join(".", @decoded_list), "\n";
}

{
    print "=========================\n";
    print "Case 3: create string from \\x{}\n";
    $encoded_string = "\x{190}\x{132}";#hex value of 400 and 306
    display $encoded_string;
    @decoded_list = unpack("U*", $encoded_string);
    print join(".", @decoded_list), "\n";
}
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