$ time perl -e 'do {open PROGRAM, "date +%s|" or die "can not open date $!";$time = <PROGRAM>;close PROGRAM} for 0 .. 1000'

real    0m14.733s
user    0m32.927s
sys     0m9.109s

$ time perl -e 'do { $time = `date +%s`} for 0 .. 1000'

real    0m14.717s
user    0m32.929s
sys     0m9.138s

$ time perl -e 'do { $time = time} for 0 .. 1000'

real    0m0.084s
user    0m0.061s
sys     0m0.046s