Without seeing more of your code, we can only speculate why you are not always catching the last payment. My best guess would be that there is a path through the code that bypasses your increment.

You might be able to simplify your code slightly as

my $numpayments = scalar keys .....
...
if (! --$numpayments) {lastpayments =1}
...
if ( $lastpayments ){ 
    print "last record\n";
};
[download]

Or even simpler

my $numpayments = scalar keys %.....
....
--$numpayments;
...
if(! $numpayments ) {
    print "Last record\n";
}
[download]

If you need to remember the $numpayments value for other uses, just make a copy and decrement that.

Using a single var and --, will eliminate a couple of possible modes of failure. It won't necessarially fix your problem, but i t might help highlight where it is.

If not, come back and post a bit more code.