#!/your/perl/here

use strict;
use warnings;

{ # closure for sub terms
  
  my @term_list = ('A'..'Z', 'a'..'z');
  
  sub terms
  {
    my $N = shift;
    my $last = 2*$N-1;
    my @pairs;
    my $index = 0;
    my $globber = '';
    while ( $index < $last )
    {
      $globber .= '{' 
               . $term_list[$index++] 
               . ',' 
               . $term_list[$index++]
               . '}';
    }
    my $terms = join ' + ', glob($globber);
    return $terms;
  }

} # end closure for sub terms

foreach my $n (1..5)
{
  my $terms = terms($n);
  print "N=$n: <", $terms, ">\n";
}

exit;

__OUTPUT__
N=1: <A + B>
N=2: <AC + AD + BC + BD>
N=3: <ACE + ACF + ADE + ADF + BCE + BCF + BDE + BDF>
N=4: <ACEG + ACEH + ACFG + ACFH + ADEG + ADEH + ADFG + ADFH + BCEG + BCEH + BCFG + BCFH + BDEG + BDEH + BDFG + BDFH>
N=5: <ACEGI + ACEGJ + ACEHI + ACEHJ + ACFGI + ACFGJ + ACFHI + ACFHJ + ADEGI + ADEGJ + ADEHI + ADEHJ + ADFGI + ADFGJ + ADFHI + ADFHJ + BCEGI + BCEGJ + BCEHI + BCEHJ + BCFGI + BCFGJ + BCFHI + BCFHJ + BDEGI + BDEGJ + BDEHI + BDEHJ + BDFGI + BDFGJ + BDFHI + BDFHJ>