This is assuming your install of v5.0 has a working POSIX.pm:

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);
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