You could use a hash of arrays

my %pats_hoa=();             
while (my $row = $sth->fetchrow_hashref){
  my $voucher   = delete $row->{'Voucher_Number'};
  my $proc_code = delete $row->{'Procedure_Code'};
  push @{$pats_hoa{$voucher}{$proc_code}},$row;
}

for my $voucher (sort keys %pats_hoa){
  for my $proc (sort keys %{$pats_hoa{$voucher}}){
    for my $rec (@{$pats_hoa{$voucher}{$proc}}){
      if ($rec->{Transaction_Type} eq '???'){
        # do something;    
      }       
    }
  }
}
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