In
http://www.utexas.edu/cola/centers/lrc/books/piep03.html
I found this passage:
"
The [ie. Holger Pedersen's] formulation [of Brugmann's rule] may be
expanded to: 'PIE short o before the consonantal allophones of the P
IE resonants became in open syllables Ind.-Ir. a:'. Thus is defined
the relation between Gk. dóru, Skt. dá:ru, Gk. pHeromes, Skt.
bhára:mas, but Gk. pósis, Skt. páti, Lat. rota, Skt. rátha. The forms
which were left unexplained even by Pedersen's formulation are clear
with the laryngeal theory : such are causative stems like janáya-,
and a in the perf. 1st sg. (there is fluctuation between a and a:
in the later language but a predominates in the old language) but
consistently long a: in the perf. 3d sg., e.g. 1st sg. cakára, 3d
sg. caká:ra. Forms like janáya- are from laryngeal bases; therefore
pre-Ind.-Ir. o in these words stood in a closed syllable.
"
It seems to me that this means that outside of the perfect, the rule
is that PIE o in a open syllable, *before a consonantal resonant*,
becomes a: in Skt., but that in the perfect, the latter condition
does not apply, only the former. Did I misunderstand something?
Torsten