use Algorithm::Combinatorics qw[ variations_with_repetition ];;

@c = variations_with_repetition( [ 1 .. 90 ], 3 );;
print scalar @c;;
729000

$d = 0; $_->[0]+1 == $_->[1] or $_->[1]+1 == $_->[2] and ++$d for @c; print $d;;
7922