johngg@abouriou:~/perl/Monks$ perl -Mstrict -Mwarnings -E 'say q{};
my @array = ( 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 4, 4, 4, 4, 4, 4, 5, 5, 6 );
my %lasts = map { $array[ $_ ] => $_ } 0 .. $#array;
say qq{$_ => $lasts{ $_ }} for sort keys %lasts;'

1 => 2
2 => 7
3 => 10
4 => 16
5 => 18
6 => 19