I'm not sure I've understood your requirement but is this what you meant?

$ perl -Mstrict -Mwarnings -MList::Util=reduce -E '
> my @arr = qw{ a c b d b a b e f };
> my %dup;
> push @{ $dup{ $arr[ $_ ] } }, $_ for 0 .. $#arr;
> my $maxKey =
>    reduce { scalar @{ $dup{ $a } } > scalar @{ $dup{ $b } } ? $a : $
+b }
>    keys %dup;
> say qq{Max dups for "$maxKey" at elements @{ $dup{ $maxKey } }};'
Max dups for "b" at elements 2 4 6
$
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I hope I've guessed correctly and that this is helpful.