use IO::Scalar;

my $stderr;
tie *STDERR, 'IO::Scalar', \$stderr;

my ($address, $timeout, $port) = ('127.0.0.1', 90, 21);
my $ftp=Net::FTP->new($address, Timeout=>$timeout, Port=>$port, Debug=>1);

## do ftp stuff..

if ($stderr)    {
    $stderr =~ s/Net::FTP.+?\(.+?\)\n?//g;  # Trim off Debug header, save meat
    $stderr =~ s/\n+/\n/g;                 # Get rid of blank lines
    open  LOG, '>>', 'some_logfile';
    flock LOG, 2;
    print LOG "$stderr\n";
    close LOG;
}