Although a documented one... basically I just want to report an interesting entry in the Journal of DAxelrod (who is also a fellow monk here), at use Perl.

To cut it short, the following:

#!/usr/bin/perl

use strict;
use warnings;

my ($num, $let, $foo);
$num = "z";
$let = $num;
# $foo = 1 + $num;  # this does not modify $num
$num++; $let++;
print "num:$num let:$let\n";

__END__
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outputs

num:aa let:aa
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whereas if you uncomment line 9, it will issue a

Argument "z" isn't numeric in addition (+) at C:\temp\foo.pl line 9.
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warning, and output

which is as of the subject, surprising and unexpected due to the fact that $num is just used, but appearently not modified in any way whatsoever. However, as the original article itself says this is both documented and useful for obfu. In this respect I was thinking that a particularly confusing effect could be achieved if one used e.g. "9" instead of "z", but that doesn't seem to be the case:

C:\temp>perl -le "$a='z';print++$a"
aa

C:\temp>perl -le "$a='9';print++$a"
10
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B::Concise seems to say the same for both:

C:\temp>perl -MO=Concise -e "$a='z'" >z
-e syntax OK

C:\temp>cat z
6  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
5     <2> sassign vKS/2 ->6
3        <$> const[PV "z"] s ->4
-        <1> ex-rv2sv sKRM*/1 ->5
4           <#> gvsv[*a] s ->5

C:\temp>perl -MO=Concise -e "$a='9'" >9
-e syntax OK

C:\temp>diff z 9
5c5
< 3        <$> const[PV "z"] s ->4
---
> 3        <$> const[PV "9"] s ->4
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(Note: damn! I couldn't use | diff z - under Windows.)

Fortunately, this is perfectly well documented as well:

The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment.

This obviously mean that "that is numeric" comprises "that looks like a number". How much it may resemble one, though, is not very intuitive. I do know it's more or less Perl's standard behaviour, but I for one would expect the string to be treated as a string in both of the following:

C:\temp>perl -le "$a='foo9';print++$a"
fop0

C:\temp>perl -le "$a='9bar';print++$a"
10
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