Perl 5 version 28.2 documentation
Returns the number of non-leap seconds since whatever time the system considers to be the epoch, suitable for feeding to gmtime and localtime. On most systems the epoch is 00:00:00 UTC, January 1, 1970; a prominent exception being Mac OS Classic which uses 00:00:00, January 1, 1904 in the current local time zone for its epoch.
For measuring time in better granularity than one second, use the Time::HiRes module from Perl 5.8 onwards (or from CPAN before then), or, if you have gettimeofday(2), you may be able to use the syscall interface of Perl. See perlfaq8 for details.
For date and time processing look at the many related modules on CPAN. For a comprehensive date and time representation look at the DateTime module.