sub truncate_utf8 {
   my ($utf8, $len) = @_;
   $len += 0;  # Make sure $len is a number.
   return $utf8 =~ s/^.{0,$len)\K(?![\x80-\xBF]).*//sr;
}

##</code><code>##

use warnings;
use strict;
use Test::More tests=>15;
use open qw/:std :utf8/;

my $in = "\N{U+CF} \N{U+2764} \N{U+1F42A}";
is truncate_utf8($in, 11), "\N{U+CF} \N{U+2764} \N{U+1F42A}";
is truncate_utf8($in, $_), "\N{U+CF} \N{U+2764} " for 10,9,8,7;
is truncate_utf8($in,  6), "\N{U+CF} \N{U+2764}";
is truncate_utf8($in, $_), "\N{U+CF} " for 5,4,3;
is truncate_utf8($in,  2), "\N{U+CF}";
is truncate_utf8($in, $_), "" for 1,0;

$in = "\xE4b";
utf8::downgrade($in); # make sure this really is a non-UTF8 string
is truncate_utf8($in, 2), "\xE4b";
is truncate_utf8($in, 1), "\xE4";
is truncate_utf8($in, 0), "";

sub truncate_utf8 {
   my ($utf8, $len) = @_;
   $len += 0;  # Make sure $len is a number.
   return $utf8 =~ s/^.{0,$len}\K(?![\x80-\xBF]).*//sr;
}

__END__

1..15
ok 1
not ok 2
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 3
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 4
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 5
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 6
#   Failed test at x.pl line 10.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084;'
not ok 7
#   Failed test at x.pl line 11.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï '
not ok 8
#   Failed test at x.pl line 11.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; '
#     expected: 'Ï '
not ok 9
#   Failed test at x.pl line 11.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084;'
#     expected: 'Ï '
not ok 10
#   Failed test at x.pl line 12.
#          got: '&#65533; '
#     expected: '&#65533;'
not ok 11
#   Failed test at x.pl line 13.
#          got: '&#65533;'
#     expected: ''
ok 12
ok 13
ok 14
ok 15
# Looks like you failed 10 tests of 15.

##</code><code>##

sub truncate_utf8 {
   my ($utf8, $len) = @_;
   return $utf8 if length($utf8) <= $len;
   my $next = substr($utf8, $len, length($utf8)-$len, '');
   $next = chop($utf8) while (ord($next) & 0xC0) == 0x80;
   return $utf8;
}

##</code><code>##

use warnings;
use strict;
use Test::More tests=>15;
use open qw/:std :utf8/;

my $in = "\N{U+CF} \N{U+2764} \N{U+1F42A}";
is truncate_utf8($in, 11), "\N{U+CF} \N{U+2764} \N{U+1F42A}";
is truncate_utf8($in, $_), "\N{U+CF} \N{U+2764} " for 10,9,8,7;
is truncate_utf8($in,  6), "\N{U+CF} \N{U+2764}";
is truncate_utf8($in, $_), "\N{U+CF} " for 5,4,3;
is truncate_utf8($in,  2), "\N{U+CF}";
is truncate_utf8($in, $_), "" for 1,0;

$in = "\xE4b";
utf8::downgrade($in); # make sure this really is a non-UTF8 string
is truncate_utf8($in, 2), "\xE4b";
is truncate_utf8($in, 1), "\xE4";
is truncate_utf8($in, 0), "";

sub truncate_utf8 {
   my ($utf8, $len) = @_;
   return $utf8 if length($utf8) <= $len;
   my $next = substr($utf8, $len, length($utf8)-$len, '');
   $next = chop($utf8) while (ord($next) & 0xC0) == 0x80;
   return $utf8;
}

__END__

1..15
ok 1
not ok 2
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 3
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 4
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 5
#   Failed test at x.pl line 9.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084; '
not ok 6
#   Failed test at x.pl line 10.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï &#10084;'
not ok 7
#   Failed test at x.pl line 11.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; &#128042;'
#     expected: 'Ï '
not ok 8
#   Failed test at x.pl line 11.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084; '
#     expected: 'Ï '
not ok 9
#   Failed test at x.pl line 11.
Wide character in print at /opt/perl5.28/lib/site_perl/5.28.1/Test2/Formatter/TAP.pm line 112.
#          got: 'Ï &#10084;'
#     expected: 'Ï '
not ok 10
#   Failed test at x.pl line 12.
#          got: '&#65533; '
#     expected: '&#65533;'
not ok 11
#   Failed test at x.pl line 13.
#          got: '&#65533;'
#     expected: ''
ok 12
ok 13
ok 14
ok 15
# Looks like you failed 10 tests of 15.