What have you tried? The general form of the thing is open your input file then enter a loop while there is more to read from the input file. In the loop open the next output file, copy n lines from the intput file to the output file, close the output file, continue with the next output file. When you exit the loop, close the input file. Where do you have difficulty with that?

split -l 2000 C prefix
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will take a file called C and split it into files called prefixaa prefixab prefixac etc., each containing 2000 lines from C. See man split for more details.

>perl -ne "BEGIN{open F,'>0.txt'}print F $_;unless($.%5){close F;open 
+F,'>'.($./5).'.txt';}" file.txt
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The filehandle will be closed when you open it again so you don't have to do that explicitly.

Here is a variation:

    perl -ne '$n=$.-1; open F, ">$n.txt" unless $n%5; print F' file
    
    Creates:
    0.txt
    5.txt
    10.txt
    ...
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This omits IO checking for the sake of a one-liner.

Here is a slightly longer one but with better output filenames:

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

    Creates:
    file000.txt
    file001.txt
    file002.txt
    ...
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Although, realistically I'd use split for this task as shown by tirwhan above.

--
John.