A question was posed to me about using substitution to perform concatenation -- more as a question as to whether substitution is flexible enough to do so rather than is this the best way to use the tools Perl provides.

So, I wrote the following as an innocuous brute force attempt: 

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;

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However, the output is: 
$ 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;
$

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Adding use strict; modifies the output to: 
$ 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;
$

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Can you help elucidate why these results are what is shown above? I see the same behavior using ActiveState 5.8.4 and 5.6 on FreeBSD 4.11.

Thanks for your candor.

In reply to concatenation through substitution? by Anonymous Monk

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