Your regex does seem to do something for me... but what does your $text contain? If you're reading it from a file, are you reading the file line-by-line or are you slurping it all at once? There are several things I could point out about the regex, like how I'm not sure what (.{0,0})\K is supposed to be doing, or that the dots . will match anything, including newlines (because of /s) and dashes. But reviewing the regex doesn't make much sense without knowing what it's supposed to match against - please provide an SSCCE with several examples of what should match and what shouldn't!

use Data::Dump;
dd "---\nFoo\n---\nBar\n"
   =~ /(.{0,0})\K(---\s*)(.{0,200})/gisx;
__END__
("", "---\n", "Foo\n---\nBar\n")
[download]

Also, note that you could set the input record separator $/ to something like "---\n" to read a file in chunks separated by that string.