Why not simply have the function return the modified date and assign it back to the variable you have passed?

sub TimeStamp($) # Pass the stamp in as YYYYMMDD
{
  local $_=$_[0];
  if( /(\d{4})(\d{2})(\d{2})/ )
  {  #returns in MMDDYYYY
        "$2/$3/$1";
  }
  else { warn "Invalid input to TimeStamp()" }
}
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and call it thusly

$date_created = TimeStamp($date_created);
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You can even use this in places like if statements.

if ( $date_created = TimeStamp($date_created) eq 'somedate' ) {
    # do something special.
}
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Also, I noticed that you are using a prototype, you might want to read this thread for reasons why you shouldn't be using them unless you have very specific reasons for doing so.