$ perl -e 'utf8::upgrade(my $u = "\x{e5}"); utf8::downgrade(my $d =
"\x{e5}"); qr{$u $d}'
Malformed UTF-8 character (1 byte, need 3, after start byte 0xe5) in
regexp compilation at -e line 1.
Malformed UTF-8 character (1 byte, need 3, after start byte 0xe5) in
regexp compilation at -e line 1.

This bug has been reported to upstream <https://rt.perl.org/rt3//Public/Bug/Display.html?id=118297> and fixed with upstream commit:

commit b8372399b387c0e276aee3d41b3c7833dd08087b
Author: Dagfinn Ilmari Mannsåker <ilmari>
Date:   Tue Jun 4 18:15:24 2013 +0100

    [perl #118297] Fix interpolating downgraded variables into upgraded regexp
    
    The code alredy upgraded the pattern if interpolating an upgraded
    string into it, but not vice versa.  Just use sv_catsv_nomg() instead
    of sv_catpvn_nomg(), so that it can upgrade as necessary.