I'm encountering a case when minmax() sometimes returns an undef max. So far I haven't been able to create a simpler test case which reproduces this problem, but this happens when I'm testing my Bencher module with Bencher::Scenario::AcmePERLANCARTestPerformance: 

% bencher -m AcmePERLANCARTestPerformance
Use of uninitialized value $max in numeric le (<=) at lib/Bencher/Form
+atter/ScaleTime.pm line 28.

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The offending code is (starting from line 25 in ScaleTime.pm): 

my ($min, $max) = minmax(map {$_->{time}} @{$envres->[2]});

my ($unit, $factor);
if ($max <= 1.5e-6) {

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Dumping the variables shows that minmax() is getting (0.002) (a single-element list with the element value of 0.002). The result from minmax is (0.002, undef).

This does not happen when the input list is of different value, e.g.: (0.0023), (0.0019), (0.0022), and so on. Only when the value is (0.002) on my computer.

This does not happen when I change the above code into: 

my @list = map {$_->{time}} @{$envres->[2]};
my ($min, $max) = minmax(@list);

...

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This does not happen when I try to reproduce this in a simple test program, using the same value of $envres.

This does not happen if I use the List::MoreUtils::PP backend (by setting LIST_MOREUTILS_PP environment variable to 1).

Any clue?

UPDATE 1: I'm now able to create a simple test to reproduce the bug. This is still List::MoreUtils 0.413. It seems related to floating point rounding in sprintf() (e.g. using %f or %g format). 

% perl -MList::MoreUtils=minmax -MData::Dump -e'for(1..20) { my ($min,
+ $max) = minmax(sprintf("%.4g", rand())); dd ($min, $max) }'
(0.9403, 0.9403)
(0.2669, 0.2669)
(0.4618, 0.4618)
(0.6728, 0.6728)
(0.829, undef)
(0.6572, 0.6572)
(0.7323, 0.7323)
(0.521, undef)
(0.03817, 0.03817)
(0.9032, 0.9032)
(0.8139, 0.8139)
(0.8573, 0.8573)
(0.9723, 0.9723)
(0.7832, 0.7832)
(0.7387, 0.7387)
(0.06714, 0.06714)
(0.127, undef)
(0.6433, 0.6433)
(0.02692, 0.02692)
(0.157, undef)

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I'm able to reproduce this on perl 5.22.1, 5.22.0, 5.20.3. perls 5.18.4 and earlier don't seem to exhibit this bug.

UPDATE 2 (2016-03-17): Bug submitted to List-MoreUtils: https://rt.cpan.org/Ticket/Display.html?id=113117 . Thanks to everyone for the responses.

In reply to List::MoreUtils' minmax bug? by perlancar

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