If you wish to gather its output and wait for the child there are two simple options. The first runs the child to completion then gets its output

perl -le '$result=`echo "hello world"`;print $result; print "OK"'
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The second starts the child and reads its output line at a time untill it is done.

perl -le 'open CHLD, "ls -l |";while(<CHLD>){chomp; print "got: $_"} p
+rint "OK"'
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Cheers,
R.