For reasons I don't know, the results can be quite different depending on the version of perl.

use strict;
use warnings;

print "Perl version $]\n";

my $str = "1 plus 2 equals 3 but in this example, AX plus BY equals CZ
+, DA plus EBCDEF plus FGH equals G and H plus I plus J plus K equals 
+L and I wonder what one gets from 7 plus Z";

my $word = "plus";

my @results = $str =~ m{ (?= (\S+ \s+ \Q$word\E \s+ \S+) ) }xmsg;

print join("\n", @results), "\n";
[download]

produces

Perl version 5.008008
1 plus 2
AX plus BY
X plus BY
DA plus EBCDEF
A plus EBCDEF
EBCDEF plus FGH
BCDEF plus FGH
CDEF plus FGH
DEF plus FGH
EF plus FGH
F plus FGH
H plus I
I plus J
J plus K
7 plus Z
[download]

or

Perl version 5.010000
1 plus 2
AX plus BY
DA plus EBCDEF
EBCDEF plus FGH
H plus I
I plus J
J plus K
7 plus Z
[download]