I also did run the tests under some configurations, and was able to reproduce some results :

Linux x86 box :
corion@samson > perl -v 
This is perl, version 5.005_02 built for i586-linux

blah blah

corion@samson >perl -w test.pl
Argument "0x1" isn't numeric in add at test.pl line 4.
01: 1
Argument "0x2" isn't numeric in add at test.pl line 4.
02: 1
Argument "0x3" isn't numeric in add at test.pl line 4.
03: 1.5
Argument "0x4" isn't numeric in add at test.pl line 4.
04: 1
... etc ...

corion@samson:~ > perl -e "print "0x30"+0;"
48corion@samson:~ >
[download]

I'm not sure what the issue with the test from the command line is, because here I use wrong double quotes which could be messed with by the shell (bash). I can confirm the other results with ActivePerl 5.005 build 517 (yes, I'm running that ancient version here), and the weird quotes trick also works under cmd.exe and NT :

F:\>perl -e "print "0x30"+0;"
48
[download]

Whatever this means, but maybe some perl luminary can enlighten us.

Update :Under Win32, ActivePerl build 517, the script works as expected, printing 0 for each number.