But you created $d and $u using "internal" Perl functions

We could debate that, but it's completely irrelevant. I used them because it made the example clear. But I could have used ordinary string literals to get the same behaviour.

can this bug be reproduced using "supported" operations?

utf8::upgrade and utf8::downgrade are fully supported. But yes.

because you're not supposed to mix codepoint-strings with byte-strings

One, there was no "mixing", so that's also completely irrelevant.

Two, that's completely untrue. "Mixing" strings with different internal storage is not only acceptable, it's common. 

use utf8;
my $u = "Éric";
my $d = "Brine";
my $s = "$u $d";  # Perfectly ok!

[download]

In reply to Re^6: how are ARGV and filename strings represented? by ikegami
in thread how are ARGV and filename strings represented? by almr

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.