As argued in The Lighter Side of Perl Culture (Part III): Obfu constraints are the heart of obfu. Some examples from that node:

• Constrain yourself to a restricted character set. For example, from A++, ${~$/&~$;&$^}++ uses only punctuation chars, while y ccccd x s vvchr oct oct ord uc ave x s vvucve le s vvuc ave x s vvchr oct oct oct ord uc bve x eval uses only alphabetic chars.
• Constrain the code to the theme of the obfu shape. For example, when tuning Saturn, I changed O^N^E}(@X,1.6) to S|A|T|U}(@R,$N) to better align the code to the Saturnian theme.
• Find an arbitrary constraint. For example, another A++ from mtve is constrained to being a palindrome: !$A+++!q!+++A$!