Use /o when

• writing for a perl 5.005 and less.
• You are nano-optimizing to avoid the null operation of a pp_regcomp given a qr// object

Use qr// when

• writing for a perl 5.6 and greater.
• You want the syntax affordance of being able to say qr/\s/ rather than "\\s" (assuming your interpolating into some larger expression)

Rely on /$regex/ implementation details when

• you've bothered to read the source and know exactly what it's doing

Do not use /o when

• writing for humans to read (because almost no one understands what /o does)
• on perls 5.6 and greater
• ever. It's just bad style now.

Do not use qr// when

• on perl 5.005 and lower
• you're writing a sub expression that's going to be interpolated elsewhere and you care that the regexp compiler takes longer to compile strings like '(?-xism:...)' slower than '...'.

Do not rely on /$regex/ implementation details when

• you haven't read the source to pp_regcomp.