encode/decode Polybius cyphers

polybius.pl

#!/usr/bin/perl

use strict;
use warnings;
use 5.010;

my $polybius;
my %polybius = (
    "a" => "11",
    "b" => "12",
    "c" => "13",
    "d" => "14",
    "e" => "15",
    "f" => "21",
    "g" => "22",
    "h" => "23",
    "i" => "24",
    "j" => "24",
    "k" => "25",
    "l" => "31",
    "m" => "32",
    "n" => "33",
    "o" => "34",
    "p" => "35",
    "q" => "41",
    "r" => "42",
    "s" => "43",
    "t" => "44",
    "u" => "45",
    "v" => "51",
    "w" => "52",
    "x" => "53",
    "y" => "54",
    "z" => "55",
);

sub help {
    return << 'HELP';

polybius.pl - encode/decode Polybius cyphers

Usage: ./polybius.pl [-d] stdin

Option:
  -d stdin	decode cypher

Note: 'i' and 'j' encode to '24', but decoding '24'
      will default to 'i'. 
      Quote multi-word strings on commandline.

HELP

}

sub encode {
    my ($str) = (@_);
    my $char;

    for my $i ( 0 .. length($str) ) {
        $char = substr( $str, $i, 1 );
        if ( exists( $polybius{$char} ) ) {
            print $polybius{$char};
        }
    }

    print "\n";
}

sub decode {
    my ($str) = (@_);
    my $pair;

    for ( my $i = 0 ; $i <= length($str) ; $i = $i + 2 ) {
        $pair = substr( $str, $i, 2 );
        foreach my $char ( keys(%polybius) ) {
            if ( $polybius{$char} eq $pair ) {
                if ( $pair eq '24' ) {
                    print "i";    # default to i
                    last;
                }
                else {
                    print $char;
                }
            }
        }
    }

    print "\n";
}

die help if ( !@ARGV );

if ( $ARGV[0] eq '-d' ) {
    shift;
    die help if ( !@ARGV );
    decode(@ARGV);
}
else {
    encode("@ARGV");
}