increment of "2\n" did not change it to a number.

Why do you say that?

13:45 >perl -Mbigint -wE "my ($m, $k) = split / /, qq[10 2\n]; say qq[
+>$k<]; ++$k; say qq[>>$k<<];"
>2
<
>>3<<

13:49 >
[download]

Update (31 Aug 14): Fixed typo: changed ++k to ++$k.

Auto-increment has here converted $k from a string to a number, as can be seen from the loss of the trailing newline. The same effect is produced by $k++ or $k += 1.

Hope that helps,

But if ++$k becomes the number 3, why it gives different answer?

print $k<$m? $m / ++$k : -1 # gives 3.(3)
print $k<$m? $m / 3 : -1    # gives 3
[download]

Yes, I think this is a bigint bug:

#! perl
use strict;
use warnings;
use bigint;

my $m = '10';
my $k =  '2';
 ++$k;
print $m / $k,    "\n";
print $k->sign(), "\n";
[download]

Output:

13:45 >perl 992_SoPW.pl
3.33333333333333
Can't locate object method "sign" via package "3" (perhaps you forgot 
+to load "3"?) at 992_SoPW.pl line 10.

13:45 >
[download]

This shows that ++ is failing to convert $k into a BigInt object. But change the line ++$k; to $k += 1; and the output is now as expected:

13:45 >perl 992_SoPW.pl
3
+

13:46 >
[download]

The documentation for bigint claims:

All operators (including basic math operations) except the range operator .. are overloaded.

But this doesn’t appear to be true for autoincrement.

Hope that helps,

Athanasius <°(((>< contra mundum

Iustus alius egestas vitae, eros Piratica,