I would craft a regex that properly gathers the entire date from the beginning of your log entry, then use something like str2time from Date::Parse to get a unix timestamp. They're much easier to do math on, and this can take care of all sorts of gotchas for you, perhaps something similar to this, depending on your log format:
my $line = 'Sep 12 15:07:48 2005 -0500 hostname rest of log entry goes
( my $date ) = ( $line =~ /^(\w\w\w \d\d \d\d:\d\d:\d\d \d\d\d\d [-|\+
print str2time( $date )."\n";
As for grabbing the last x number of lines from a file, you could open a pipe to a tail:
open (PIPE, "/path/to/tail -n 10 /path/to/logfile |");
Update: Better yet, try File::Tail.