A more funcional approach:

use List::MoreUtils qw( pairwise );

my @arr = qw( J K L M N O );

my %done;
my @pairs = grep !$done{$_}++,       # Remove duplicates
            pairwise { "$a - $b" }
            @arr;

print map "$_\n", @pairs;
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The above is basically the same as:

my @arr = qw( J K L M N O );

my %done;
my @pairs;
while (@arr) {
    my $pair = join(' - ', splice(@arr, 0, 2));
    next if $done{$pair}++;
    push @pairs, $pair;
}

print "$_\n" for @pairs;
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