The intent of this function is to merge two, two dimensional arrays and return the merged array.  The first and second elements (array indicies 0 and 1, respectivly) are left alone in the merge because the first is text and the second is what I key off of (munging my keys would generally be considered a Bad Thing and text does not play well with mathematical operators).

sub merge
{
  my @a;
  my @b;
  {
    my $a_ref = shift;
    my $b_ref = shift;
    @a = @{$a_ref};
    @b = @{$b_ref};
  }
  my %hash;
  my @c;

  for(my $x = 0; $x <= $#b; $x++) { $hash{$b[$x][1]} = \@{$b[$x]};}
  for(my $x = 0; $x <= $#a; $x++)
  {
    if((exists $hash{$a[$x][1]}) && (defined $hash{$a[$x][1]}))
    {
      for(my $y = 2; $y <= $#{$hash{$a[$x][1]}}; $y++)
        { $hash{$a[$x][1]}[$y] = ($hash{$a[$x][1]}[$y] + $a[$x][$y]) /
+ 2; }
    }
    else { $hash{$a[$x][1]} = \@{$a[$x]}; }
  }
  foreach (sort { $a <=> $b } keys %hash) { push @c, \@{$hash{$_}}; }

  return @c;
}
[download]

Update: Removed prototype.