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

print join("\n", @results), "\n";

Perl version 5.008008
1 plus 2
...
I plus J
J plus K
7 plus Z

Perl version 5.010000
1 plus 2
...
I plus J
J plus K
7 plus Z