Normal files are seekable. Net::FTP filehandles do a "greedy" seek.
Problematic when you want to read from a specific offset.
my $filehandle = $ftp->retr('remote_ftp_file.dat');
# seeking ahead will fail... because standard seek is greedy
# instead:
$filehandle->sysseek(2**5);
while (<$filehandle>) {
# ladida...
}