Well, you could easily test that theory yourself. ;o) But unfortunately no. You must not insert the (7) into the format, you must overwrite existing characters instead.
Step 1: Write the format you want.
Step 2: Force Perl6::Form to validate the format length at runtime.
In both cases above the string length is identical before and after the addition of this validation number.
And really, most of the time you should just skip step 2. It doesn't buy you much. You can check these things with a decent text editor. As the author said, it was primarily meant for use by automated tools.