5.1. Write a program to read lines from all of the files on the command line and print out each line prefaced with its filename. (Answer to Exercise 5.1)

5.2. Modify your answer for Exercise 5.1 so after it reads lines from the files you specify on the command line, it reads in lines from standard input and prefaces the line with stdin. (Answer to Exercise 5.2)

5.3. Explain the output of this program then fix the bug. The line with while comes straight from the perlfaq4 documentation. You can see the whole answer about adding commas to numbers using perldoc –q comma:

#!/usr/bin/perl
use strict;
use warnings;
 
print format_number( 9999 ), "\n";

sub format_number {
    local $_ = shift;

    1 while s/^([-+]?\d+)(\d{3})/$1,$2/;

    print $_;
    }