use Data::Dumper;

my @group1 = ('A','B','C','D','E');
my @group2 = ('F','G','H','I','J');

foreach my $hit (@group2) {
    push (@group1, $hit) unless grep { $_ eq $hit } @group1;
}

print Dumper( \@group1 );

##</code><code>##

$VAR1 = [
          'A',
          'B',
          'C',
          'D',
          'E',
          'F',
          'G',
          'H',
          'I',
          'J'
        ];