The others have already given you some ideas for better parsing. However, you're mistaken on the premise of the question:

It then occurred to me that the code won't run if the regexp has no capture groups! ... I need the captures.

The code will still run - a regex in list context without /g and without capture groups will return the list (1) if it matched, so the assignment will evaluate to true. See also.

use warnings;
use strict;
use Data::Dump;

my @syntax = ( [qr/cd/, sub { dd "callback", \@_ }] );
my $line = "abcdef";

for my $syn (@syntax) {
    my ($re, $cb) = @$syn;
    if (my (@matches) = ($line =~ $re)) {
        $cb->(@matches);
        last;
    }
}

__END__

("callback", [1])
[download]

Update: And $#+ will give you the number of capture groups present in the last successful match (see also).