Get out early with Perl statement modifiers

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When I first start­ed writ­ing Perl in my ear­ly 20’s, I tend­ed to fol­low a lot of the struc­tured pro­gram­ming con­ven­tions I had learned in school through Pascal, espe­cial­ly the notion that every func­tion has a sin­gle point of exit. For example:

sub double_even_number {
    # not using signatures, this is mid-1990's code
    my $number = shift;
    if (not $number % 2) {
        $number *= 2;
    }
    return $number; 
}

This could get pret­ty con­vo­lut­ed, espe­cial­ly if I was doing some­thing like val­i­dat­ing mul­ti­ple argu­ments. And at the time I didn’t yet grok how to han­dle excep­tions with eval and die, so I’d end up with code like:

sub print_postal_address {
    # too many arguments, I know
    my ($name, $street1, $street2, $city, $state, $zip) = @_;
    # also this notion of addresses is naive and US-centric
    my $error;
    if (!$name) {
        $error = 'no name';
    }
    else {
        print "$name\n";
        if (!$street1) {
            $error = 'no street';
        }
        else {
            print "$street1\n";
            if ($street2) {
                print "$street2\n";
            }