perl -ne '$n=$.-1; open F, ">$n.txt" unless $n%5; print F' file
    
    Creates:
    0.txt
    5.txt
    10.txt
    ...


##</code><code>##

    perl -ne 'BEGIN{$s="file000"} open F, ">". $s++. ".txt" unless ($.-1)%5; print F' file

    Creates:
    file000.txt
    file001.txt
    file002.txt
    ...