sub write {
    @_ == 3 or croak 'Usage: $sftp->write($fh, $data)';

    my ($sftp, $rfh) = @_;
    $sftp->flush($rfh, 'in') or return undef;

    if (utf8::is_utf8($_[2])) {
        splice(@_, 2, 1, "$_[2]");   # Don't affect caller.
        utf8::downgrade($_[2], 1)  # Change format to bytes.
            or carp("Can only write bytes to a socket");
    }
    ...

##</code><code>##

use strict;
use warnings;

use Encode qw( encode );

my $enc = 'iso-latin-1';  # Desired encoding
my $text = chr(130);   # Any char supported by iso-latin-1 and $enc

utf8::downgrade(my $text_off = $text);
utf8::upgrade  (my $text_on  = $text);

# Encode text on output
my $from_off = encode($enc, $text);
my $from_on  = encode($enc, $text);
print("bytes are ", ($from_off eq $from_on ? 'same' : 'diff'), "\n");

##</code><code>##

bytes are same

##</code><code>##

use strict;
use warnings;

utf8::downgrade( my $bytes_off = '' );
$bytes_off .= chr($_) for 0..255;
utf8::upgrade( my $bytes_on = '' );
$bytes_on .= chr($_) for 0..255;

# Downgrade variable on output to avoid false positive.
utf8::downgrade( my $from_off = $bytes_off );
utf8::downgrade( my $from_on  = $bytes_on );
print("bytes are ", ($from_off eq $from_on ? 'same' : 'diff'), "\n");

##</code><code>##

bytes are same