You don't need to know about the guts, but you do need to encode your data, if you care about its encoding.
Unfortunately, you do in some circumstances. For example, the only difference in between $up and $dn in the following is the internal storage format, yet they differ in output.
$\ = "\n";
utf8::downgrade $dn = chr(0xC9);
utf8::upgrade $up = chr(0xC9);
# <5.10 >=5.10
print pack('A', $dn) eq chr(0xC9) ?1:0; # 1 1
print pack('A', $up) eq chr(0xC9) ?1:0; # 0 1
print $dn =~ /\w/ ?1:0; # 0
print $up =~ /\w/ ?1:0; # 1
They are being fixed. As you can see, pack no longer depends on the internal encoding since 5.10.0. Other discrepencies such as /\w/ are being fixed too.