andromeda:davidj perl_test > cat f.txt
^this^
^is^
^a^
^test^
^david#jenkins^
^ cinea#jenkins ^

##</code><code>##

andromeda:davidj perl_test > cat out.txt
^this^
^is^
^a^
^test^
^ddaavviidd#jjeennkkiinnss^
^  cciinneeaa#jjeennkkiinnss  ^

##</code><code>##

#!/usr/bin/perl

use strict;

open(FILE, "<f.txt");
open(OUT, ">out.txt");
while(<FILE>) {
    my $str = "";
    chomp $_;
    if( 1 .. 4 ) {
        print OUT "$_\n";
        next;
    }
    while( $_ =~ m/(.)/g ) {
        if( $1 =~ m/(\^|\#)/ ) {
            $str .= "$1";
        } else {
            $str .= "$1$1";
        }
    }
    print "$str\n";
    print OUT "$str\n";
}
close(FILE);
close(OUT);

##</code><code>##

#!/usr/bin/perl

use strict;

open(FILE, "<f.txt");
open(OUT, ">out.txt");
while(<FILE>) {
    chomp $_;
    if( 1 .. 4 ) {
        print OUT "$_\n";
        next;
    }

    for( my $i = 0; $i < length($_); $i++ ) {
        if( substr($_, $i, 1) =~ m/(\^|\#)/ ) {
            substr($_, $i, 1) = "$1";
        } elsif( substr($_, $i, 1) =~ m/(.)/ ) {
            substr($_, $i, 1) = "$1$1";
            $i++;
        }
    }
    print OUT "$_\n";
}
close(FILE);
close(OUT);

##</code><code>##

andromeda:davidj perl_test > perl test.pl
              Rate 2nd string   In place
2nd string 28969/s         --       -17%
In place   35112/s        21%         --