my %filter_map = ( ... );

my $filter = q{};
if ($filter_clean) {

    my @filter_split = split //, $filter_clean;

    for my $fc (@filter_split) {
        if ( $fc =~ /\d/ ) {
            $filter .= $fc;
        }
        else {
            $filter .= $filter_map{ uc $fc };
        }
    }
}

say $filter;