use strict;
use warnings;

my @a = (1, 3, 5, 6, 7, 8);
my @b = (2, 3, 5, 7, 9);

my (@isect, %seen, %isect);

foreach my $e (@a) { $seen{$e} = 1 }

foreach my $e (@b) {
    if ( $seen{$e} ) { $isect{$e} = 1 }
}

@isect = keys %isect;

print "@isect\n";