Execute a series of commands in PSQL
If you are using psql 9.6+, there is a very convenient command called \gexec:


  Sends the current query input buffer to the server, then treats each
  column of each row of the query's output (if any) as a SQL statement
  to be executed. [...] The generated queries are executed in the order
  in which the rows are returned, and left-to-right within each row if
  there is more than one column. NULL fields are ignored. The generated
  queries are sent literally to the server for processing, so they
  cannot be psql meta-commands nor contain psql variable references.


So, your script could look like:

SELECT format('create database slave%s template master_db', l_num) 
  FROM generate_series(1, 30) t(l_num);

\gexec


Note that you can use the newer psql version, even if the server version is lower.  There might be some rare cases of incompatibility (when using \d* commands, because some system views may change between server versions), but I haven't yet seen such a case.

If you are still stuck on an earlier client version, you can just use the SELECT and copy the output manually to a script file.

If your database names are in a file, you have multiple options.  Either treat that file with an editor (something like sed -i -e 's/^/CREATE\ DATABASE\ /' -e 's/$/\ TEMPLATE\ master_db' in any OS where sed exists), and then run the resulting script.

Alternatively, you can also do something like this:

CREATE TEMPORARY TABLE db_names (db text);

\copy db_names FROM 'path/to/your/file'

SELECT format('create database slave%s template master_db', db) 
  FROM db_names;

\gexec


Important note: only use \gexec when you absolutely trust the source you are working with.  The above approach is prone to SQL injection (which, interestingly, not so easy when one of the commands on the same line is CREATE DATABASE, but it's always better to be careful than sorry later).