You'll need to pass a reference to the scalar variable as a parameter to the subroutine.

TimeStamp(\$data_created);

sub TimeStamp {
  my ($dateRef) = @_;
  if ($$dateRef =~ /(\d{4})(\d{2})(\d{2})/) {
    $$dateRef = "$2/$3/$1";
  } else {
    warn("...");
  }
}
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Since Perl has call-by-value semantics, i.e. a copy of the variable is passed to the subroutine rather than the value itself, you have to pass the address of the variable (i.e. \$date_created) to the subroutine.

In the subroutine, you manipulate the value $$dateRef which is stored at address $dataRef.

Hope this helps, -gjb-