die 'insufficient arguments' unless @ARGV == 1;
open(IN, $ARGV[0]) or die "unable to open '$ARGV[0]'";
while (my $line = <IN>) {
    my $prefix = sprintf '%4d', $.;
    $line =~ s/$line/$prefix: $line/;
    print $line;
}
close IN;

##</code><code>##

$ perl test.pl test.pl
   1: die 'insufficient arguments' unless @ARGV == 1;
open(IN, $ARGV[0]) or die "unable to open '$ARGV[0]'";
while (my $line = <IN>) {
    my $prefix = sprintf '%4d', $.;
    $line =~ s/$line/$prefix: $line/;
    print $line;
   7: }
   8: close IN;
$

##</code><code>##

$ perl test.pl test.pl
   1: use strict;
   2: die 'insufficient arguments' unless @ARGV == 1;
open(IN, $ARGV[0]) or die "unable to open '$ARGV[0]'";
while (my $line = <IN>) {
    my $prefix = sprintf '%4d', $.;
    $line =~ s/$line/$prefix: $line/;
    print $line;
   8: }
   9: close IN;
$