#!/usr/bin/perl

use warnings;


# Define the subroutine
sub linenum
{
	my @new_array;		# This is not necessary in Perl but makes the variables
	my $n = 0;		# local to avoid conflicts in the rest of the program
	foreach (@_)
	{
		$n++;
		push @new_array, "$n " . $_;		
	}
	@new_array;
}

# Grab the file names off the command line, input file first then output file
$input_file = $ARGV[0];
$output_file = $ARGV[1];

# This is a more compact and idiomatic way of opening a file (see pages 167-168)
open INPUT, "<$input_file" or die "Can't open $input_file for reading\n";
@input_array = <INPUT>;			# This stores the entire file in memory
close INPUT;				# and will not be efficient with LARGE files

@output_array = linenum(@input_array);	# Call the subroutine

open OUTPUT, ">$output_file" or die "Can't open $output_file for writing\n";
print OUTPUT @output_array;
close OUTPUT;

# Note that "_array" appended to the names are redundant
# They can be identified as arrays with the @ symbol
# Although whatever is easier to read should be used