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my $str = "It/PRP really/RB does/VBZ seem/VB to/TO violate/VB a/DT lot
+/NN of/IN boundaries!/NNS";

...
for (0..$#pairs-1) {
    print $pairs[$_][0], " ", $pairs[$_+1][0], "\n";
}

$nth = 2; # every 2nd words
for (0..$#pairs-$nth) {
    print $pairs[$_][0], " ", $pairs[$_+$nth][0], "\n";
}