Not sure what "undef" does though.

The undef in your do call is for attributes to pass to the do method (I've never seen anyone use those but I guess it may apply if you want an attribute passed to the prepare method) - see DBI.

I also dont understand why putting it into a variable makes it work...

The difference between what you were doing (which did not work) and what you are doing is you were passing you insert data in the SQL string before and now you are passing it as a parameter:

before "insert into mytable (x) values($street)" results in insert into mytable(x) values(high street)" which will not parse correctly because of the unquoted "high street".

now you are using insert into mytable(x) values(?)" which does parse correctly and the DBD passes the $street variable as a bound parameter which does not need quoting (as it is not in the SQL string) and the driver takes all of the string as the replacement parameter.