use POSIX 'strftime';

# replace with actual date
my @localtime = qw/YYYY MM DD 00 00 00/;

# see perldoc -f localtime
$localtime[0] -= 1900;
$localtime[1] -= 1;

print strftime('%V', reverse @localtime);
[download]

From my copy of (perldoc POSIX):

If you want your code to be portable, your
format ("fmt") argument should use only the conversion
specifiers defined by the ANSI C standard. These are
"aAbBcdHIjmMpSUwWxXyYZ%".
[download]

I'm not familiar with the apparently non-standard %V code. Does that offer the "week number" as requested by the original poster? On what platform(s)?

--
[ e d @ h a l l e y . c c ]

Yes, it corresponds to ISO's standard definition of a week number.

Tested on:
Linux 2.4.21, Perl v5.6.1
Linux 2.6.10, Perl v5.8.3
Mac OS X Tiger 10.4.2, Perl v5.8.6

Of course, if the OP is still using v5.0 I've got no idea what kind of old OS they're using, so YMMV.

1. some code I found by googling:
   
   Read more... (2 kB)
2. Install modules somewhere, where you have write access, for example in your homedirectory or somewhere else. You then just need to tell your Perl program where to find those modules:
   Read more... (904 Bytes)
   hth,
   Sven