Vigenère cipher

#!/usr/bin/perl
use warnings;

#Input arguments
our $key = $ARGV[0];    #REDDIT
our $text = $ARGV[1];   #TODAYISMYBIRTHDAY

#Possible Values
our %abc =
(
    A => 0,
    B => 1,
    C => 2,
    D => 3,
    E => 4,
    F => 5,
    G => 6,
    H => 7,
    I => 8,
    J => 9,
    K => 10,
    L => 11,
    M => 12,
    N => 13,
    O => 14,
    P => 15,
    Q => 16,
    R => 17,
    S => 18,
    T => 19,
    U => 20,
    V => 21,
    W => 22,
    X => 23,
    Y => 24,
    Z => 25
);

#Process arguments
our @text_chars = ();
our @key_chars = ();
our @crypted_chars = ();

#Read arguments
&process_arguments($key, $text);

sub process_arguments(){

    #Get Key Length
    @key_chars = split(//, $_[0]);
    my $key_length = scalar @key_chars;

    #Get Text Length
    @text_chars = split(//, $_[1]);
    my $text_length = scalar @text_chars;


    #Extend Key to Text Length
    while($key_length < $text_length){
        push(@key_chars, @key_chars);
        $key_length = scalar @key_chars;
    }

    #Turn hash around
    my %numbers = reverse %abc;

    #Get crypted chars
    my $i = 0;
    for ($i = 0; $i < $text_length; $i++) {
        my $a = $abc{uc($key_chars[$i])};
        my $b = $abc{uc($text_chars[$i])};

        my $sum_mod = ($a+$b)%26;
        $crypted_chars[$i] = $numbers{$sum_mod};
    }

    print @crypted_chars;

}