I wonder if this code is introducing a bit of a off-by-one problem: 

C4::Circulation::checkHighHolds(), line 1230-1234

     # If the number of holds is less than the count of items we have
     # plus the number of holds allowed above that count, we can stop here
     if ( $holds->count() <= $threshold ) {
         return $return_data;
     }

The comment says "less than" but the code says "less than or equal".

A couple of tests seem to be failing for me:

$ sudo koha-shell -c "prove -v t/db_dependent/DecreaseLoanHighHolds.t" kohadev
t/db_dependent/DecreaseLoanHighHolds.t .. 
1..17
ok 1 - userenv set
ok 2 - Static mode should exceed threshold
ok 3 - Should have 5 outstanding holds
ok 4 - Should have duration of 1
ok 5 - due_date should be a DateTime object
ok 6 - New due hour is equal to original due hour.
ok 7 - New due minute is equal to original due minute.
ok 8 - New due date second is zero.
not ok 9 - Should not exceed threshold

#   Failed test 'Should not exceed threshold'
#   at t/db_dependent/DecreaseLoanHighHolds.t line 142.
#          got: '1'
#     expected: '0'
ok 10 - Should exceed threshold of 1
not ok 11 - Should not exceed threshold of 2

#   Failed test 'Should not exceed threshold of 2'
#   at t/db_dependent/DecreaseLoanHighHolds.t line 160.
#          got: '1'
#     expected: '0'
ok 12 - Should exceed threshold with one damaged item
ok 13 - Should exceed threshold with one lost item
ok 14 - Should exceed threshold with one notforloan item
ok 15 - Should exceed threshold with one withdrawn item
ok 16 - High holds checkout needs confirmation
ok 17 - High holds checkout does not need confirmation
# Looks like you failed 2 tests of 17.
Dubious, test returned 2 (wstat 512, 0x200)
Failed 2/17 subtests 

Test Summary Report
-------------------
t/db_dependent/DecreaseLoanHighHolds.t (Wstat: 512 Tests: 17 Failed: 2)
  Failed tests:  9, 11
  Non-zero exit status: 2
Files=1, Tests=17, 12 wallclock secs ( 0.02 usr  0.01 sys +  9.69 cusr  0.70 csys = 10.42 CPU)
Result: FAIL

I really don't understand the logic of the CheckHighHolds sub:

sub checkHighHolds {
    my ( $item, $borrower ) = @_;
...
    if ( $holds->count() ) {
...
        if ( $decreaseLoanHighHoldsControl eq 'static' ) {
...
            # If the number of holds is less than the threshold, we can stop here
            if ( $holds->count() < $decreaseLoanHighHoldsValue ) {
                return $return_data;
            }
        }
        elsif ( $decreaseLoanHighHoldsControl eq 'dynamic' ) {
...
            # If the number of holds is less than the count of items we have
            # plus the number of holds allowed above that count, we can stop here
            if ( $holds->count() <= $threshold ) {
                return $return_data;
            }
        }
...
        if ( DateTime->compare( $reduced_datedue, $orig_due ) == -1 ) {
            $return_data->{exceeded} = 1;
            $return_data->{duration} = $decreaseLoanHighHoldsDuration;
            $return_data->{due_date} = $reduced_datedue;
        }
    }

    return $return_data;
}

As far as I can tell, it does the right thing for decreaseLoanHighHoldsControl = static/dynamic, but then it goes and calculates the regular/original due date and the decreased due date, and if the decreased date is less than the original date, then the decreased date is returned? Why is this done?