$ cat use.pl 
#!/usr/bin/env perl 
use strict;
use warnings;
use POSIX;
use Date::Format;
$ ./use.pl 
Prototype mismatch: sub main::strftime: none vs ($\@;$) at /usr/share/
+perl5/vendor_perl/Exporter.pm line 66.
 at ./use.pl line 5.
Prototype mismatch: sub main::ctime: none vs ($;$) at /usr/share/perl5
+/vendor_perl/Exporter.pm line 66.
 at ./use.pl line 5.
Prototype mismatch: sub main::asctime: none vs (\@;$) at /usr/share/pe
+rl5/vendor_perl/Exporter.pm line 66.
 at ./use.pl line 5.
$ perl -v | head -3

This is perl 5, version 20, subversion 3 (v5.20.3) built for x86_64-li
+nux-thread-multi
(with 16 registered patches, see perl -V for more detail)
[download]

I would hypothesise that because the prototypes are different, the subroutines are not actually redefined. Just a guess, though.