But ...

> Adding use utf8; was one of the first things I tried.

... if your source-code is in utf8 (check your editor settings) and you have a line like my $t='abc...едц'; you must apply use utf8;

Otherwise Perl will not know how to decode the bytes in that string, because the interpretation is not obvious.

You should clarify this, before meddling with XML.

Here a demo you should run:

use v5.12;
use warnings;
use Data::Dump;


my $t1='едц';
ddx $t1;
say "length: ",length $t1;

use utf8;

my $t2='едц';
ddx $t2;
say "length: ",length $t2;
[download]

# demo_utf8.pl:8: "\xC3\xA5\xC3\xA4\xC3\xB6"                <-- bytes
length: 6
# demo_utf8.pl:14: "\xE5\xE4\xF6"                           <-- code p
+oints
length: 3
[download]