# 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;

##</code><code>##

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