sub bp_count
{
    my ($data) = @_;

    # obtain the count of each nucleotide in the sequence
    my $a_count = ($$data =~ tr/Aa//);
    my $c_count = ($$data =~ tr/Cc//);
    my $g_count = ($$data =~ tr/Gg//);
    my $t_count = ($$data =~ tr/Tt//);

    return ($a_count, $c_count, $g_count, $t_count);
}

# Called with:
bp_count(\$string);