my @qfns;
my @fhs;

for my $qfn (@qfns) {
   open(my $fh, '<', $qfn)
      or die("Can't open \"$qfn\": $!\n");
   push @fhs, $fh;
}

for (;;) {
   my @lines;
   my $all_done  = 1;
   my $some_done = 0;
   for my $fh (@fhs) {
      push @lines, my $line = <$fh>;
      if (defined($line)) { $some_done = 1; }
      else                { $all_done  = 0; }
   }

   exit(0) if $all_done;
   die("Premature eof\n") if $some_done;

   chomp @lines;
   print(join(' ', @lines), "\n");
}