If the code is not terribly complex, you can always use join. If it is, consider pulling out the last element (with pop) and saving it for after the loop. You will most likely want to encapsulate the body of the loop into a subroutine. Something like this:

my $last = pop @tables;

foreach my $table (@tables) {
    process($dbh,$table);
    print ",\n";
}

process($dbh,$last);
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L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)