Changing variables in place is usually a bad idea. It can be hard to debug and to modify, and it's a bit inflexible. I would rather write the code as such:

my $formatted_date = time_stamp( $date_created )
    or warn "Invalid date: '$date_created'";

sub time_stamp
{
    my $date = shift;
    return "$2/$3/$1" if $date =~ /(\d{4}(\d{2})(\d{2})/;
}
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