So instead of scripting, he could have generated a sorted list of numbers from the files he had. Created a file with the sequence of numbers for the range and diffed/commed the whole thing. Voilà...

    $ seq -w 0001 0003
    0001
    0002
    0003
    $

    $ seq -w 98 102
    098
    099
    100
    101
    102

  seq -f %04g 0 3
  echo {0000..0003} # if bash

    import sys, pathlib
    basedir = pathlib.Path(sys.argv[1])
    for i in range(1, 501):
        if not (basedir / f'{i:04}_A.csv').is_file():
            print(i)

    for k in $(seq -f %04.f 1 501); do 
        f="${k}_A.csv"
        ! [[ -f "$f" ]] && echo $f || :
    done

    for k in {0001..0501}_A.csv; do
        ! [[ -f "$f" ]] && echo $f || :
    done

    parallel -kj1 '! [[ -f "{}" ]] && echo {} || :' ::: {0001..0501}_A.csv

    $ echo -e '0001\n0010\n0002\n0' | bc
    1
    10
    2
    0

$ printf "%d\n" 003

3

    $ printf "%d\n" 009
    sh: 1: printf: 009: not completely converted
    0

I love when programs mysteriously fail when passed an IP address like 192.168.001.009.

Thankfully modern tools have been moving away from supporting that notation and are less likely to explode all over unsuspecting users.

$ printf "%1.0f\n" 009

9

$ printf "%1.0f\n" 00125990

125990

  $ printf %d 010
  8

  ls ????_A.csv | grep -o '[1-9][0-9]*'

Then again, Perl is even scarier.

But yeah, the perl rabbit hole goes as deep as you want to, and in some sense it makes it more difficult to say "screw it, I'm redoing this in a real language" as complexity rises. And then you end up with a thousand lines of line noise.

That said the Perl that you write in a one-liner should be different than the Perl that you write when the complexity rises.

If you have more than a few hundred lines, you probably need to move some of it out into a module. There should also be tests for that module. It might even make sense to structure it like a CPAN module so that you can use the existing tools to test and install it.

I wouldn’t wanna use it for more than short scripts. Perl 6 might be fun, but it doesn’t seem to have a large enough community.

If you want to improve your Perl code read “Modern Perl”. (There is more than one version, and I know the first version was made freely available online)

Perl is a better language for large codebases than most people give it credit for. That said it allows more creativity when it comes to your code. So you can make awful code just as easily as beautiful code. Perl6 makes the beautiful code easier to write and shorter, while making awful code a bit harder to write.

Good news! Every version is freely available online. Here's the most recent:

http://modernperlbooks.com/books/modern_perl_2016/index.html