my $start_time = time;
END { my $exec_time = time - $start_time;
      print("Program took $exec_time seconds to execute\n"); }

...[ put rest of program here ]...
[download]

or if you want to include compile time

my $start_time;
BEGIN { $start_time = time; }
END   { my $exec_time = time - $start_time;
        print("Program took $exec_time seconds to execute\n"); }

...[ put rest of program here ]...
[download]

Neither will include the time it took to load Perl itself.
Add use Time::HiRes qw( time ); if you want higher precision results.