To be honest, I would get your users to create CSV files with whatever spreadsheet software you normally use.

If you want them to create CSV files on-the-fly from within your code, I'd take a look at Text::CSV.

If you really have a requirement to roll-your-own code for this, which I think would be highly unlikely, the guts of it might look something like the following.

$ ls -l pm_user_input_file
ls: pm_user_input_file: No such file or directory
[download]

$ perl -Mstrict -Mwarnings -E '
    use autodie qw{:all};
    open my $fh, q{>}, q{pm_user_input_file};
    while (<>) {
        print {$fh} $_;
    }
'
1,2,3
q,w,e
a,s,d
z,x,c
[download]

$ ls -l pm_user_input_file
-rw-r--r--  1 ken  staff  24 19 Jun 23:49 pm_user_input_file
[download]

$ cat pm_user_input_file
1,2,3
q,w,e
a,s,d
z,x,c
[download]

Again, just so you are clear on this, unless you are doing this purely out of personal interest or for some similar reason, this is a really poor option which I'd advise you avoid!