Weird perl errors on OS X 10.10 (Yosemite)

Can't locate strict.pm: Permission denied at ...

You run some command involving Perl and...:

$ ack foo
Can't locate strict.pm:   Permission denied at /usr/local/bin/ack line 14.
BEGIN failed--compilation aborted at /usr/local/bin/ack line 14.

or you try to install something funky with homebrew and you got some weird Permission denied errors on automake-something.pm. Apparently nobody else on the internet is experiencing the same problem.

Given that I sadly remember almost nothing about Perl I had to look up how to print Perl's include directories:

$ perl -wle'print for @INC'
/Library/Perl/5.18/darwin-thread-multi-2level
/Library/Perl/5.18
/Network/Library/Perl/5.18/darwin-thread-multi-2level
/Network/Library/Perl/5.18
/Library/Perl/Updates/5.18.2
/System/Library/Perl/5.18/darwin-thread-multi-2level
/System/Library/Perl/5.18
/System/Library/Perl/Extras/5.18/darwin-thread-multi-2level
/System/Library/Perl/Extras/5.18
.

to find out the directory with the wrong permissions:

$ ls /Library/Perl/Updates/5.18.2
ls: /Library/Perl/Updates/5.18.2: Permission denied
$ sudo ls -ld /Library/Perl/Updates/
drwxr-x---  3 root  wheel  102 19 Ott 12:38 /Library/Perl/Updates/

On a Linux system I would have used strace, but on OS X things are a little different:

# dtruss -f 'su MYUSER -c perldoc perlre' 2>&1 | grep -C 3 Permission
...
66887/0x34fda:  stat64("/Network/Library/Perl/5.18/Pod/Perldoc.pm\0", 0x7FFF51E0C960, 0x2000)            = -1 Err#2
66887/0x34fda:  stat64("/Library/Perl/Updates/5.18.2/Pod/Perldoc.pmc\0", 0x7FFF51E0CA10, 0x2000)                 = -1 Err#13
66887/0x34fda:  stat64("/Library/Perl/Updates/5.18.2/Pod/Perldoc.pm\0", 0x7FFF51E0C960, 0x2000)          = -1 Err#13
66887/0x34fda:  write(0x2, "Can't locate Pod/Perldoc.pm:   Permission denied at /usr/bin/perldoc5.18 line 10.\nBEGIN failed--compilation aborted at /usr/bin/perldoc5.18 line 10.\n\004\b\0", 0x95)                 = 149 0
...

As you can see from the result of the stat64(2) calls there are two different type of errors (Err#2 and Err#13), one for a non-existent file and one for two files which our user can't access. The first stat64 call checks for a compiled version (.pmc) of the Perldoc library for which a permission denied error is negligible, but the same error on the .pm file is an hard error, the one which I was getting at the beginning.