steph_bow,
The naive way to do this might look like: 
#!/usr/bin/perl
use strict;
use warnings;
while (<DATA>) {
    chomp;
    my @col = split /;/;
    push @col, '' for @col .. 2;
    print join ';', @col;
    print "\n";
}
__DATA__
a;3
b;2;3
c;5;4
d;6

[download]
A less naive solution would use a module from CPAN such as Text::CSV or Text::xSV. To understand why that is a better approach, take a few minutes with Super Search.

Update: As noted by tye in the CB, this solution was originally not tested. I added chomp and changed @col .. 3; to @col .. 2;

Cheers - L~R

In reply to Re: adding ";" to void elements by Limbic~Region
in thread adding ";" to void elements by steph_bow

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