You're close.

unless ( $tax_collection =~ /^[a-zA-Z]+(?:,[a-zA-Z]+)*?$/ ) {
    print "<font color=\"#ff0000\"><i>Incorrect format</i></font>";
    $errors++;
}
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What that does is it says match [a-zA-Z] as many times as possible (the first word) followed by a sequence that can be repeated as many times as necessary (or no times). That sequence may start with a comma, and finish with as many [a-zA-Z] characters as possible. The match is anchored from the front of the string to the end (assuming a single-line string).