#!/usr/bin/perl -w
use strict;

require utf8;

my $s= pack "C*", 1..255;   # Byte string to convert

my $u= pack "U*", 1..255;   # UTF-8 string
my $e= substr($u,0,0);      # Empty UTF-8 string

my $r= $s;                  # Convert using regex
$r =~ s{
    ([^\0-\x7F])
}{
    my $o= ord($1);
    sprintf
        "%c%c",
        0xc0 | ( $o >> 6 ),
        0x80 | ( $o & 0x3f );
}gex;

my $i= $s.$e;               # Convert by implicit upgrade to UTF-8

my $f= $s;                  # Upgrade via utf8.pm function
utf8::upgrade( $f );

my $b= $s;                  # Upgrade then mark as bytes
utf8::encode( $b );

if(  $r eq $b  ) {
    print "The regex and utf8::encode() match.\n";
}
if(  $u eq $i  &&  $i eq $f  ) {
    print "The 3 Unicode strings match.\n";
}
if(  join(" ",unpack"C*",$r) eq join(" ",unpack"C*",$i)  ) {
    print "The byte- and unicode-strings have the same bytes.\n";
}
if(  $r ne $i  ) {
    print "The byte- and unicode-strings are not equal.\n";
}
print '$s contains ', length($s), " bytes.\n";
print '$i contains ', length($i), " characters.\n";
print '$r contains ', length($r), " bytes.\n";

##</code><code>##

The regex and utf8::encode() match.
The 3 Unicode strings match.
The byte- and unicode-strings have the same bytes.
The byte- and unicode-strings are not equal.
$s contains 255 bytes.
$i contains 255 characters.
$r contains 383 bytes.