Ugh, I'm so silly. Even after I learned what /o does at YAPC, from Dominus. I0's example answers the question.

As for what /o does, it means "do not compile this again." What it does get that done is this:

print "trying...\n";
$x = qr/japhy/o;
print $you =~ $x;

# makes a tree like

[print "trying\n"]
[compile regex /japhy/]
[return regex (some internal form)]
[match regex on $you]
[print]

# the /o modifier tells [return regex] to MODIFY the tree

[print "trying\n"] -------------------\
   [compile regex /japhy/]            |
[return regex (some internal form)] <-/
[match regex on $you]
[print]
[download]

That's what happens. The op-tree is modified because of the /o modifier. Perl never sees the "compile this regex" op again, and instead, it uses some pre-compiled regex.

japhy -- Perl and Regex Hacker