For adjacent entries you don't have to iterate over the array twice:

use warnings;
use strict;

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

for (my $i = 0; $i < @a; $i += 2){
    my $str = $a[$i] . ' ' . $a[$i+1];
    print $str, "\n";
    $done{$str} = 1;
}
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In this case %done isn't really used, if you want each pair to be printed only once you can check if $done{$str} is already set.