use strict;
use warnings;
use 5.10.1;

my $filter_clean = 'GAV18';
my %filter_map = (
    A => 2,
    B => 2,
    # ...
    Y => 9,
    Z => 9,
);
$filter_map{$_} = $_ for 0..9;

(my $output = uc $filter_clean)
    =~ s{ (.) }{ $filter_map{$1} }exg;

say $output;