In this context @files returns the number of elements in the array @files. This ternary operator expression (@files > 6 ? 6 : @files) works like this:

     my $number_to_submit=@files;
    
     if @files > 6 then $number_to_submit = 6; 
                   else $number_to_submit = @files;
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So this limits the number of files to submit to 6 if there are more than 6 files in the array @files.