Presumably, the last three digits are msecs. Try simply cutting them off (divide by 1000):

use POSIX;

my $epoch = 1260439520772;
my $date = strftime '%Y/%m/%d %H:%M:%S', localtime $epoch/1000;
print "date=$date\n";

__END__
date=2009/12/10 11:05:20
[download]