There are regexp pattern modifiers (m, s, i and x).
There are regexp quote operator (qr//) modifiers (m, s, i, x, p and o).
There are match operator (m//) modifiers (m, s, i, x, p, o, g and c).
There are substitution operator (s///) modifiers (m, s, i, x, p, o, g, c and e).

perlre discusses regexp patterns, so it only mentioned regexp pattern modifiers. perlop mentions the operator modifiers along with the operators they modify.

By the way, you should have no reason to use the 'o' modifier now that qr// exists.

Update: It used to be that only the 'm', 's', 'i' and 'x' modifiers were listed in perlop. Now I see some of the operator modifiers are there too. 'o' was surely omitted because it's obsoleted by qr//, like I previously mentioned.

perlre for 5.6.0: msix
...
perlre for 5.6.2: msix
perlre for 5.8.0: msix
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perlre for 5.8.8: msix
perlre for 5.8.9: msixpgc
perlre for 5.10.0: msixpgc