use 5.020;
# Heuristically "fix" a broken string
use strict;
use warnings;

use utf8;
use Encode qw/encode decode/;

my $chars      = 'абвд';
my $bytes = encode('UTF-8',$chars);

my $mixed_pickles = "$chars $bytes $chars $bytes";
say "Before: ", encode('UTF-8',$mixed_pickles);

my $utf8_decodable_regex =
    qr/[\xC0-\xDF][\x80-\xBF]    | # 2 bytes unicode char
       [\xE0-\xEF][\x80-\xBF]{2} | # 3 bytes unicode char
       [\xF0-\xFF][\x80-\xBF]{3}/x;

$mixed_pickles =~ s/($utf8_decodable_regex)/
    decode('UTF-8',$1,Encode::FB_CROAK | Encode::LEAVE_SRC)/gex;

say "After:  ", encode('UTF-8',$mixed_pickles);