Why does POSIX in conjunction with Win32 throw a warning with floor()?

You're correct, both POSIX and Win32 export, by default, NULL.

POSIX is a poorly behaved module that exports far, far, far too much by default (list at the bottom). To account for this, only import the functions you need.

use Win32;
use POSIX qw(floor);

POSIX uses Exporter to accomplish this. See How To Import for more detail about controlling what gets imported.

$ perl -wle 'use POSIX; print join ", ", @POSIX::EXPORT'

isupper, isspace, fabs, F_GETLK, strncpy, EBADMSG, localeconv, SIGTRAP, ctermid, S_ISUID, fwrite, pow, strcoll, S_ISBLK, _POSIX_STREAM_MAX, EACCES, putc, FILENAME_MAX, tolower, sinh, EMLINK, ESOCKTNOSUPPORT, EDESTADDRREQ, DBL_MIN, fopen, TOSTOP, strncat, LINK_MAX, ENXIO, INLCR, TCION, NAME_MAX, EINPROGRESS, SIGILL, NDEBUG, VEOF, SEEK_END, ungetc, SEEK_CUR, STDOUT_FILENO, VEOL, ftell, UINT_MAX, ENOTEMPTY, DBL_EPSILON, INPCK, WIFSIGNALED, B134, remove, LC_TIME, SIGSEGV, _POSIX_PATH_MAX, F_RDLCK, SIG_BLOCK, VINTR, SA_NOCLDSTOP, PATH_MAX, isdigit, log10, O_RDWR, ENOTCONN, TMP_MAX, signal, F_SETLKW, qsort, O_TRUNC, _SC_TZNAME_MAX, _POSIX_NGROUPS_MAX, LC_COLLATE, _PC_NO_TRUNC, SCHAR_MAX, EHOSTUNREACH, fputs, ctime, fgetc, O_APPEND, _POSIX_ARG_MAX, EWOULDBLOCK, TCSAFLUSH, strstr, _exit, execle, malloc, DBL_MANT_DIG, _POSIX_SSIZE_MAX, puts, _SC_JOB_CONTROL, ttyname, B150, EMFILE, CS6, _POSIX_LINK_MAX, asin, mblen, _POSIX_PIPE_BUF, sigsuspend, B600, SIGPROF, L_ctermid, _SC_CLK_TCK, ceil, ECHILD, tmpfile, isprint, ECHOE, memset, ENOLINK, atexit, MAX_CANON, EADDRINUSE, sigprocmask, stderr, fscanf, modf, setpgid, tcgetpgrp, toupper, ENETRESET, B2400, raise, S_ISDIR, _SC_PAGESIZE, DBL_MAX_EXP, sysconf, EIDRM, F_SETFD, O_NOCTTY, EHOSTDOWN, FLT_MAX, CSTOPB, S_IRWXU, EPROTO, TCSANOW, S_IRWXO, setbuf, strchr, strerror, FLT_MIN_EXP, TCIOFF, tan, SIGCONT, EDQUOT, MB_CUR_MAX, _PC_PATH_MAX, SIGTTOU, SIGXCPU, EROFS, fdopen, _PC_VDISABLE, CHILD_MAX, ETXTBSY, S_ISCHR, SIGTTIN, VERASE, ESRCH, LONG_MAX, mbtowc, pause, sscanf, MB_LEN_MAX, O_WRONLY, fstat, _PC_MAX_INPUT, F_SETLK, SIGHUP, S_IXUSR, ETIME, DBL_MAX_10_EXP, execvp, ENOTSOCK, DBL_MIN_10_EXP, TCSADRAIN, isalnum, getchar, EMSGSIZE, TCIOFLUSH, _SC_NGROUPS_MAX, FLT_RADIX, ENOTDIR, _PC_LINK_MAX, strspn, S_IRWXG, _POSIX_NO_TRUNC, EXIT_SUCCESS, VKILL, acos, ERESTART, vprintf, EPFNOSUPPORT, IGNCR, _PC_MAX_CANON, STDIN_FILENO, strxfrm, _SC_VERSION, isxdigit, setsid, _POSIX_NAME_MAX, fmod, VSTART, B9600, FLT_MANT_DIG, islower, EXIT_FAILURE, clock, ENETDOWN, CS7, strrchr, SIGUSR2, tcdrain, INT_MIN, LDBL_DIG, _POSIX_JOB_CONTROL, SIG_UNBLOCK, _SC_STREAM_MAX, X_OK, F_UNLCK, ETIMEDOUT, CHAR_BIT, tmpnam, W_OK, sigpending, cfgetospeed, IEXTEN, geteuid, SIGRTMAX, E2BIG, LDBL_MIN, _SC_CHILD_MAX, CLK_TCK, NCCS, tzset, ENOMEM, gets, BRKINT, EDOM, ENODATA, ENOBUFS, ISTRIP, CLOCKS_PER_SEC, LDBL_MIN_EXP, SHRT_MIN, PARODD, EOF, asctime, ENFILE, EPROCLIM, freopen, sigaction, F_DUPFD, O_ACCMODE, FLT_MAX_10_EXP, difftime, TCOFLUSH, EINTR, ENOMSG, L_cuserid, B4800, EAGAIN, TCOON, setjmp, TZNAME_MAX, S_IWOTH, cuserid, PIPE_BUF, strtol, HUGE_VAL, F_GETFD, IGNPAR, EBUSY, memmove, ENOTBLK, getgid, SIGINT, EUSERS, SIGURG, EDEADLK, EOWNERDEAD, creat, _POSIX_MAX_CANON, _POSIX_CHOWN_RESTRICTED, execlp, F_SETFL, stdout, SIG_DFL, ldiv, SIGKILL, VSUSP, ENOTRECOVERABLE, B300, B200, HUPCL, WTERMSIG, offsetof, clearerr, tanh, getcwd, LDBL_MAX_10_EXP, SIG_SETMASK, ECHONL, O_NONBLOCK, S_IXOTH, ECONNABORTED, F_OK, tcflush, _POSIX_SAVED_IDS, SIGPIPE, _PC_NAME_MAX, ECANCELED, SIGCHLD, EREMOTE, FLT_MAX_EXP, SEEK_SET, getpid, B1800, NOFLSH, SIGUSR1, ECONNRESET, wcstombs, ESPIPE, WSTOPSIG, rewind, BUFSIZ, SIGABRT, STREAM_MAX, vsprintf, tcsendbreak, LDBL_MIN_10_EXP, pathconf, S_IRGRP, _SC_SAVED_IDS, OPOST, execv, feof, O_EXCL, access, sigsetjmp, mktime, fread, B1200, LC_MESSAGES, EXDEV, S_IROTH, longjmp, SA_RESETHAND, LC_ALL, ENOSYS, calloc, B110, FLT_EPSILON, assert, VQUIT, B50, ICANON, IXON, ECONNREFUSED, strftime, _PC_PIPE_BUF, ERANGE, SA_ONSTACK, ispunct, _POSIX_MAX_INPUT, WIFSTOPPED, ldexp, ENOLCK, EOTHER, _PC_CHOWN_RESTRICTED, PARENB, O_CREAT, STDERR_FILENO, ARG_MAX, ETOOMANYREFS, isatty, S_ISFIFO, SIGQUIT, abort, EPIPE, isalpha, USHRT_MAX, SA_RESTART, bsearch, IGNBRK, stdin, EPROTONOSUPPORT, ENOSPC, fgets, getegid, EAFNOSUPPORT, setvbuf, SIGTSTP, getuid, ESHUTDOWN, LONG_MIN, fgetpos, _POSIX_VERSION, frexp, %SIGRT, EADDRNOTAVAIL, F_WRLCK, lseek, EISDIR, atol, cfsetospeed, SIGALRM, fpathconf, B38400, L_tmpname, _POSIX_OPEN_MAX, ESTALE, LC_CTYPE, S_ISREG, WIFEXITED, EPROTOTYPE, SIG_IGN, EIO, ENAMETOOLONG, EPERM, atoi, isgraph, ENOENT, errno, MAX_INPUT, setuid, _SC_OPEN_MAX, S_IRUSR, siglongjmp, getenv, CS8, EINVAL, NULL, ECHO, LDBL_EPSILON, SCHAR_MIN, ENETUNREACH, uname, DBL_MAX, ENOPROTOOPT, SIGSTOP, strtoul, SA_NODEFER, CREAD, SIGBUS, mbstowcs, EFBIG, cfsetispeed, ISIG, FLT_MIN, SA_NOCLDWAIT, fsync, LDBL_MAX_EXP, ENOTTY, VMIN, strtod, TCIFLUSH, SA_SIGINFO, fclose, strcspn, strpbrk, SIGTERM, ENOSTR, ULONG_MAX, LC_NUMERIC, scanf, getgroups, vfprintf, ENOSR, FLT_ROUNDS, EEXIST, S_IWGRP, ENOEXEC, SIGVTALRM, SIGPOLL, memcmp, atan, putchar, _POSIX_CHILD_MAX, fflush, fsetpos, WEXITSTATUS, atof, EFAULT, memchr, strcat, VSTOP, _POSIX_TZNAME_MAX, LDBL_MAX, strlen, setlocale, FLT_MIN_10_EXP, cosh, tcgetattr, realloc, div, CHAR_MAX, fprintf, UCHAR_MAX, execve, B75, ICRNL, strcpy, ECHOK, FD_CLOEXEC, cfgetispeed, iscntrl, strtok, SSIZE_MAX, SIGSYS, S_ISGID, strncmp, EISCONN, labs, CLOCAL, R_OK, memcpy, F_GETFL, VTIME, dup, EALREADY, fseek, strcmp, SIGXFSZ, dup2, wctomb, SHRT_MAX, SIGFPE, SIG_ERR, _SC_ARG_MAX, setgid, execl, RAND_MAX, CSIZE, tcflow, CS5, LC_MONETARY, TCOOFF, _POSIX_VDISABLE, PARMRK, perror, mkfifo, ENODEV, S_IXGRP, WNOHANG, ferror, WUNTRACED, floor, INT_MAX, EOPNOTSUPP, OPEN_MAX, LDBL_MANT_DIG, DBL_DIG, SIGRTMIN, CHAR_MIN, tzname, O_RDONLY, B0, tcsetattr, tcsetpgrp, ELOOP, EOVERFLOW, S_IWUSR, IXOFF, EILSEQ, DBL_MIN_EXP, ENOTSUP, EBADF, B19200, free, fputc, NGROUPS_MAX, FLT_DIG