#!/usr/bin/perl
use 5.016;
use Data::Dumper;
# 1030798

my $str = "chapter 1,2,3";     # out: "chapter 1, chapter  2, chapter 3"
my @arr = split / /,$str;      # split to 2 element array, "chapter" & "1,2,3"

my @chaptnums = split /,/,$arr[1];
for my $elem(@chaptnums) {
	print "$arr[0] $elem";      # e.g. "chapter " . num
	if (scalar $elem != (1+$#chaptnums) ) {  # not the last @chaptnums element?
		print ", ";
	} else {
		print "\n";
	}
	
}

=head EXECUTION

C:>1030798.pl
chapter 1, chapter 2, chapter 3

=cut