Typing "dist://perl" into the PerlMonks Search box (top of this page) takes you to perl. You can select release 10.0 from a drop-down there and Goto that version and then use "Other tools" to grep the source or "browse".

Line 199 is the first line of:

void
recursive_lock_release(pTHX_ recursive_lock_t *lock)
{
    MUTEX_LOCK(&lock->mutex);
    if (lock->owner == aTHX) {
        if (--lock->locks == 0) {
            lock->owner = NULL;
            COND_SIGNAL(&lock->cond);
        }
    }
    MUTEX_UNLOCK(&lock->mutex);
}
[download]

from http://cpansearch.perl.org/src/RGARCIA/perl-5.10.0/ext/threads/shared/shared.xs.

http://cpansearch.perl.org/src/RGARCIA/perl-5.10.0/thread.h contains:

#  define MUTEX_LOCK(m) \
    STMT_START {                        \
    int _eC_;                        \
    if ((_eC_ = pthread_mutex_lock((m))))            \
        Perl_croak_nocontext("panic: MUTEX_LOCK (%d) [%s:%d]",    \
                 _eC_, __FILE__, __LINE__);    \
    } STMT_END
[download]

So 22 is "eC" (error code, I'd guess). A $! of 22 is "Invalid argument" for a random Linux box I checked. You should be able to grep for pthread_mutex_lock to dig deeper, etc.

I hope that helps.