## untested code follows ...

my($max,$min) = @array1 > @array2 ?
  (\@array1, \@array2)
:
  (\@array2, \@array1);

my @keys = qw/ year month day hhmm /;
my @newarray;

for my $i (0 .. @$max) {
  last if $i >= @$min;

  if(@keys == grep { $array1[$i]->{$_} eq $array2[$i]->{$_} } @keys) {
     push @newarray => {
       %{ $array1[$i] },
       data2 => $array2[$i]->{data2}
     };
  } else {
     push @newarray => $array1[$i], $array2[$i];
  }
}