my $n = 0;
while ( $str1 =~ /(\S+\s+)(?=(\S+\s+\S+))/g ) {
   print "$t1$t2\n";
   ++$n;
}

##</code><code>##

my @words = split ' ', $str;
my $n = @words - 2;
print "@words[$_+0, $_+1, $_+2]\n"
   for 0..$n-1;