If u can use other commands than system()..u may use open() like this:

open (logHdle, ">C:/path2log.txt");
open (progHdle, "ping somehost|");

while(<progHdle>)
{
    print logHdle $_ ;
    print STDOUT $_;
}
close(logHdle);
close(progHdle);
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I havent tested the code, but it should work