Here's a cute little program that will turn a file of text upside-down (sort of): 
#!/usr/bin/perl
use strict;
use warnings;

my @lines;

while(<>){
 push @lines, $_;
}

while(@lines){
 print flip_line(pop @lines);
}

sub flip_line 
{rev_string(join '', map flip_char($_), split //, shift)}

sub rev_string 
{scalar reverse shift}

sub flip_char 
{$_= lc shift; tr/'ahbbmfnnjpdrvutwqgye'/'eyqgwtuvrdpjnnfmbbha'/; $_}

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For example, the file 
some text can be
flipped over using
this script

[download]
becomes 
fdijcs siyf
buisn jano paddilt
aq uec fxaf awos

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Flipping the text back over can be approximated by something like 
./umop-apisdn.pl < test.txt | ./umop-apisdn.pl

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This is approximate because of case changes and because the mapping function I use doesn't invert cleanly (since v and u both man to n, for example).
Note: this idea is not originally mine, I just wrote this implementation. Check out http://poignantguide.net/ruby/chapter-7.html for the inspiration.
~dewey

Replies are listed 'Best First'.
Re: Turning text umop-apisdn
by jwkrahn (Abbot) on Mar 27, 2007 at 09:36 UTC
    Why are you using tr/// to change a single quote to a single quote -- twice? Why split and join the line? Why not just: 
    for ( reverse <> ) {
    chomp;
    ( my $line = reverse lc ) =~ tr/ahbbmfnnjpdrvutwqgye/eyqgwtuvrdpjn
+nfmbbha/;
    print "$line\n";
    }

    [download]
[reply]
[d/l]
      Thanks for the clean-up. I didn't have a good understanding of all the different things reverse can do (in particular, the 
      for ( reverse <> ) {

      [download]
      usage is new to me). Splitting and joining... oh, that was left-over from when I thought I needed to split up the string in order to reverse it (again, I just don't know what reverse does, I'd better hit perldoc) and I just spaced on getting rid of those steps.
      Not sure that you mean by using tr/// twice...
      ~dewey
[reply]
[d/l]

        He means that you do not need to quote the arguments to tr:

        tr/ahbbmfnnjpdrvutwqgye/eyqgwtuvrdpjnnfmbbha/

        Good Day,
            Dean

[reply]
[d/l]

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