Thanks to both crenz and runrig.

I followed up on the leads crenz presented here. He is right. In the get() of Net::FTP, it calls syswrite to write to the local file, and that's the whole problem.

Now it is clear that syswrite obviously does not work with in-mem.

I changed the syswrite to a print. As I don't want that impact the normal get(), I added one function called get_in_mem() to Net::FTP on my PC.

I simply replicated get(), changed that syswrite to print, and striped some code that is obviously not neccessary for opened handlers (as the original get() also deals with filenames).

Here is the code (obviously I didn't fully test it, but for my case, I tested it and it worked):

sub get_in_mem {
    my($ftp,$remote,$local,$where) = @_;

    my($loc,$len,$buf,$resp,$data);
    local *FD;

    my $localfd = 1;

    croak("Bad remote filename '$remote'\n") if $remote =~ /[\r\n]/s;

    ${*$ftp}{'net_ftp_rest'} = $where if ($where);

    delete ${*$ftp}{'net_ftp_port'};
    delete ${*$ftp}{'net_ftp_pasv'};

    $data = $ftp->retr($remote) or return undef;

    $loc = $local;
    
    if($ftp->type eq 'I' && !binmode($loc)) {
        carp "Cannot binmode Local file $local: $!\n";
        $data->abort;
        return undef;
    }

    $buf = '';
    my($count,$hashh,$hashb,$ref) = (0);

    ($hashh,$hashb) = @$ref if($ref = ${*$ftp}{'net_ftp_hash'});

    my $blksize = ${*$ftp}{'net_ftp_blksize'};

    while(1) {
        last unless $len = $data->read($buf,$blksize);

        if (trEBCDIC && $ftp->type ne 'I')
        {
            $buf = $ftp->toebcdic($buf);
            $len = length($buf);
        }

        if($hashh) {
            $count += $len;
            print $hashh "#" x (int($count / $hashb));
            $count %= $hashb;
        }
        print $loc $buf;
=document
        my $written = syswrite($loc,$buf,$len);
        unless(defined($written) && $written == $len)
        {
            carp "Cannot write to Local file $local: $!\n";
            $data->abort;
            close($loc)
            unless $localfd;
            return undef;
        }
=cut
    }

    print $hashh "\n" if $hashh;

    unless ($data->close()) # implied $ftp->response
    {
        carp "Unable to close datastream";
        return undef;
    }

    return $local;
}
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