perl -pe 'tr/a-zA-Z/n-za-mN-ZA-M/;'

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;

die "Run with argument." unless defined $ARGV[0];
my $file = $ARGV[0];

if ( $file =~ /\.enc/ ) {
    decrypt( $file );
} else {
    encrypt( $file );
}
exit;

sub encrypt {
    my $f = shift;
    my $enc = $f . '.enc';
    files( $f, $enc );
}

sub decrypt {
    my $f = shift;
    my $dec = $f;
    $dec =~ s/\.enc$/.dec/;
    files( $f, $dec );
}

sub files {
    my ( $in_file, $out_file ) = @_;
    open my $in, '<', $in_file or die "Cannot read input file $in_file : $!\n";
    open my $out, '>', $out_file or die "Cannot write to output file $out_file : $!\n";
    rotate( $in, $out );
    close $in;
    close $out or die "Failed to complete write to output file $out_file : $!\n";
}

sub rotate {
    my ( $fh_in, $fh_out ) = @_;
    while ( my $text = <$fh_in> ) {
        $text =~ tr/a-zA-Z/n-za-mN-ZA-M/;
        print { $fh_out } $text;
    }
}


##</code><code>##

$ perl foo foo
$ perl foo foo.enc
$ diff foo foo.dec
$