How portable do you need this to be? On unix, this oughta work, but is dirty:

my $date_fmt = grep s/^date_fmt="(.*)"$/$1/, `locale -k LC_TIME`;

You could do this in c (one hopes) on any POSIX system — possibly inlining it in Perl with Inline::C — with the setlocale() function; make the second paramater NULL to query the current locale.