Very simple -- you want to concatenate the strings instead:

my $_retVal = system $ENV{ ComSpec }. '/c'. $_fullCmd ;
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I think your mistake was that you're used to printing strings that way (ie. by separating arguments with commas), but for system, of course, you need a concatenated string.

Update:  After I thought about that for a minute, I realized it shouldn't make a difference.  But of course someone else had already beaten me to that conclusion.  Please ignore my erroneous advice above, and give your ++ votes to runrig instead.  :-(