$ split --lines=1000 --numeric-suffixes=1 --suffix-length=5 --additional-suffix=NR.txt /the/file ''

##</code><code>##

</the/file perl -MIO::File -ne'
    my $nr = 1+int($./1_000);
    $handles[$nr] = IO::File->new(sprintf("%05dNR.txt", $nr), "w") unless defined $handles[$nr];
    $handles[$nr]->print($_);
'