If you don't mind clobbering the array you could just:

use warnings;
use strict;

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

print join (' - ', splice @arr, 0, 2), "\n" while @arr;
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Prints:

J - K
L - M
N - O
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or if you need the unique check:

use warnings;
use strict;

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

while (@arr) {
    my $str = join ' - ', splice @arr, 0, 2;
    next if exists $done{$str};
    
    $done{$str}++;
    print "$str\n";
}
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