The tag you are looking for is <code> or <c> - wrap your entire source block in this and it will likely produce the result you are looking for. See Writeup Formatting Tips and/or Markup in the Monastery.

Assuming your label syntax follows the following prototype:

LOOP_A: for (my $i = 0; $i < @array1; $i++) {
    LOOP_B: for (my $j = 0; $j < @array2; $j++) {
        LOOP_C: for (my $k = 0; $k < @array3; $k++) {
            print "$i, $j, $k\n";
            next LOOP_A if $j == $k and $i == $k;
        }
    }
}
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you do not have a syntax error in your labeling. Post the sample code where perl cannot locate the label, and we'll help you see the issue.

Side note: I am guessing from your for condition that you are using C-style for loops and not Perl-style for loops. It's not an error, but they are more error prone and require more typing.