foreach my $paymentRow ( @$payments ) {
  foreach my $salesKey ( keys %$sale_items ) {
     ### make sure we're applying the right line items ...
     next if ( $paymentRow->{creditacct} != 
        $sale_items->{$salesKey}{debitacct} );

     ## if payment can be applied, 
     ## apply it and delete the row from payments/credits array
     warn "DELETING ...  {$salesKey} " 
       if ( $paymentRow->{amount} == 
              $sale_items->{$salesKey}{amount} and
            $paymentRow->{ppid} == 
               $sale_items->{$salesKey}{ppid} );
         
     delete $sale_items->{$salesKey}      <---- DELETED
       and undef( $paymentRow ) 
       if ( $paymentRow->{amount} == 
              $sale_items->{$salesKey}{amount} 
            and $paymentRow->{ppid} == 
              $sale_items->{$salesKey}{ppid} );
               
     $sale_items->{$salesKey}{amount} -= 
        $paymentRow->{amount}  
      and undef( $paymentRow )
        if ( $paymentRow->{amount}  < 
             $sale_items->{$salesKey}{amount}   <---- AUTOVIVI
           and $paymentRow->{ppid} == 
               $sale_items->{$salesKey}{ppid} );
        }
   }
[download]

as demonstrated in

$salesKey = 'key';
$sales_items = { $salesKey => {} };

     delete $sale_items->{$salesKey};

print("[", keys(%$sale_items), "]\n");  # []

     doodah()
        if ( $paymentRow->{amount}  < 
             $sale_items->{$salesKey}{amount});

print("[", keys(%$sale_items), "]\n");  # [key]
[download]

To fix, change

     delete $sale_items->{$salesKey}
       and undef( $paymentRow ) 
       if ( $paymentRow->{amount} == 
              $sale_items->{$salesKey}{amount} 
            and $paymentRow->{ppid} == 
              $sale_items->{$salesKey}{ppid} );
[download]

to

     do {
       delete $sale_items->{$salesKey};
       undef( $paymentRow );
       next;
     } if ( $paymentRow->{amount} == 
              $sale_items->{$salesKey}{amount} 
            and $paymentRow->{ppid} == 
              $sale_items->{$salesKey}{ppid} );
[download]

or start the last if with $sale_items->{$salesKey} and.