Since it's possible the flag doesn't matter, the snippet you posted should probably be changed to the following, the fatal equivalent of "Wide character in print". 

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");
    }
    ...

[download]

Try finding out if the flag is on for a good reason

I don't see how that's relevant.

If the data is text, it doesn't matter if the flag is on or not when you encode the data as needed.

If the data isn't text, it doesn't matter if the flag is on or not. Just use utf8::downgrade. 

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");

[download]
bytes are same

[download]
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");

[download]
bytes are same

[download]

Update: Fleshed out some details. Changed to use $_[2] as in original snippet.

In reply to Re^2: UTF8 error when using Net::SFTP::Foreign by ikegami
in thread UTF8 error when using Net::SFTP::Foreign by deadpickle

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.