Well ... first you pass in a parameter and then you modify a global variable?

Another problem is that you modify $_ without localizing it!

The $_ = $_[0]; made a copy of the parameter and you then modified the copy, not the parameter itself. You either want to work on the $_[0] directly or use

sub TimeStamp {
  for ($_[0]) {
    if( /(\d{4})(\d{2})(\d{2})/ ) {
      #returns in MMDDYYYY
      $_ = "$2/$3/$1"
    } else {
      warn "Invalid input to TimeStamp()";
    }
  }
}
...
TimeStamp($date_created); # will modify $date_created
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Jenda