#!/usr/bin/perl
use 5.016;
# 1030798ALT

my $str = "chapter 1,2,3";     # out: "chapter 1,chapter  2,chapter 3"
my ($chapter, @num);

if ( $str =~ /^(chapter) / ) {
    $chapter = $1;
    # say "\$chapter: |$chapter|";
}

my @chapnums = ( $str =~ /(\d[,]*)/gc);
    if ( @chapnums) {
        push @num,@chapnums;
    }

for my $num(@num) { 
    # say "DEBUG   \$#num: $#num";
    # if ( $1 != (1 + $#num) ) {
    #     print "\$chapter $num" . ",";
    # } elsif ( $1 == (1 + $#num) ){
    print "$chapter $num";
}
[download]

output: chapter 1,chapter 2,chapter 3

Less plodding, perhaps less mad, but still a longform version of suggestions above.

virudinesh: your title and narrative don't agree about whether you want a comma after "chapter 3" -- an internal contradition that suggests carelessness on your part, and makes it difficult for the Monks to offer a definitive answer. It's not that doing it either way is hard; it's that it's hard to know what you want, and thus, what help you need.