The problem is that you messed up byte context with character context. Just adding one line, your demo code can be easily fixed to demo the right result:

#!/usr/bin/perl -lw

$a="à"; # A high latin1 character, doesn't even need unicode

print '$a Normal substr: ', ord(substr($a,0,1));
{
    use bytes;
    print '$a Bytes  substr: ', ord(substr($a,0,1));
}
{
    use bytes;#I added this
    $b = $a . chr(256); 
}
chop $b;
print '$a equals $b, but $b is internally in UTF8' if $a eq $b;

print '$b Normal substr: ', ord(substr($b,0,1));
{
    use bytes;
    print '$b Bytes  substr: ', ord(substr($b,0,1));
}
[download]

This gives:

$a Normal substr: 224
$a Bytes  substr: 224
$a equals $b, but $b is internally in UTF8
$b Normal substr: 224
$b Bytes  substr: 224
[download]

update after read thospel's reply:

thospel, my point is not to argue with you about the encoding or representation. The point is that, you tried to use your demo to disapprove "use bytes", but it actually did the opposite, and proved "use bytes" is alright. In your case, the first byte of $a and $b are different, and Perl did printed different ord, so it proved that "use bytes" is just fine.

All what the OP asked is how to safely get the first byte, and "use bytes" is one of the correct way to do it. I just don't get how your big lesson on encoding is related to the original question. By reading the original post, to me, the author does not sounds like someone has no idea about all the encoding stuff, my feeling is that he knows quite a lot, otherwise he would not even ask the right question.

Your demo on "use bytes" simply cannot be used to disapprove "use bytes", and is misleading in general.