CSS-Tiny tries to reduce load time and memory overhead on web servers.
CSS.pm requires about 2.6 MB of RAM to load, which is a large amount of overhead if only wanting to do trivial things, but memory usage is normally scoffed at in Perl.
This module is primarily for reading and writing simple files, and anything we write shouldn't need to have documentation/comments.
With the increasing complexity of CSS, this is becoming more common, but many situations can still live with simple CSS files.
CSS::Tiny supports grouped styles of the form this, that { color: blue } correctly when reading, ungrouping them into the hash structure. However, it will not restore the grouping should you write the file back out.