Use the g regex modifier to capture all the pairs. Count them and see how many you have. Then decide. Use the non-capturing parenthesis (?: ) for the space. This still leaves a lot of questions - e.g. what to with character triples?

while (<>){
   my @charpairs = /(?: \s+ (\w\w) ) /gx;
   if($charpairs != 4){
          print "Funny looking at line $. :\n\t'$_'\n"}
   else{
      #do smthing
    }}
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throop