s{^
   ( #capture
     \d{2} #MM
   / \d{2} #DD
   / \d{4} #YYYY
   \s+
     \d{2} #HH
   : \d{2} #MM
   : \d{2} #SS
   ) #stop capturing
  \. \d{3} #XXX
$}
{$1}x #replace by captured