# I didn't copy the top of your file, and you should add use Data::Dumper; if you want this code to work
my $header = $csv->getline($FH);
my @keys = @$header;
my %result;

while (my $row = $csv->getline($FH))
{
  my %currentRow;
  @currentRow{@keys} = @$row;
  print "Current row: ", Dumper \%currentRow;
  my $uniqueKey = "$currentRow{Amount} & $currentRow{AmountId} & $currentRow{MRP}"; # Bad idea if '&' is allowed in either of those values
  if(!exists $result{$uniqueKey}) # If this combinaison is unknown
  {
    $result{$uniqueKey} = [\%currentRow]; # save the current row to it
  }
  else
  {
    push @{ $result{$uniqueKey} }, \%currentRow; # add the current row to the ones with the same ID
  }
}

print "Result: ", Dumper \%result;