That won't work if the term is longer than one character...

True, true.

Actually, my preference is for an approach like that of Re: Building a boolean search engine. It's more wordy, but also provides more flexibility and control.
(Update: But I guess one could ask: If you're going to do all that, why not just write a recursive descent parser?)

So, something like:

# use feature ':5.10';

use strict;
use warnings;

my $str = "In this example, AA plus B equals C,
D minus EE times FFF equals G and HH plus
I times JJ minus K equals L and
M plusplus N plus plus O is invalid and
P equals Q and RRR plus T";

use constant OPS => qw(plus minus times);

my $op     = qr{ \b (?: @{[ join '|', map quotemeta, OPS ]} ) \b }xms;
my $not_op = qr{ (?! $op) }xms;

my $operand = qr{ \b (?: $not_op \w)+ \b }xms;

my $term = qr{ $operand \s+ $op \s+ $operand }xms;

my @terms = $str =~ m{ (?= ($term)) }xmsg;

print "$_ \n" for @terms;
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Output (same for ActiveState 5.8.2 and Strawberry 5.10.0.5):

AA plus B
D minus EE
EE times FFF
HH plus
I
I times JJ
JJ minus K
RRR plus T
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