# store files in arrays
open(A,'file_a');
my @a = (<A>);
close(A);

open(B,'file_b');
my @b= (<B>);
close(B);

# now the fun obfu bit
my $max =  [ @b + 0 => @a + 0 ] -> [ @b + 0 <= @a + 0 ];

for (0 .. $max) {
  print $a[$_] . $b[$_];
}