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$ time perl -e 'print "start program. \n"; exec ("time ls -lR /usr | w +c");' start program. ... real 0m9.012s user 0m1.339s sys 0m3.929s
$ time perl -e 'print "start program. \n"; exec ("time ls -lR /usr | w +c &");' start program. ... real 0m4.102s user 0m1.200s sys 0m2.963s