It will be slower, but since you are using Mason anyway you are presumably using mod_perl. So the cost of loading the subroutines through the autoloader will only be paid the first time a given subprocess loads the module.

You should probably load this module (any any other module you use a lot) in the mason_handler itself. In my handler I have:

{
  package HTML::Mason::Commands;

  # Used by modules
  use Apache::DBI;
  use DBI;
  use DBD::Oracle;
  use CGI qw(-compile :all *table);
}
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The braces are significant because they provide the appropriate scope for the package so the subroutines get imported into the correct namespace. You need to make sure that your package lines up with the one that mason will run your code in (HTML::Mason::Commands is the default so use that unless you overrode it in the Parser).

The -compile argument to the 'use CGI' forces all subroutines to be loaded now rather than on their first usage so that they load into shared pages of the base apache process and the memory will be shared across all children. So you pay a little up front cost when starting the server, but subsequent uses are pretty fast and you save memory. What a deal

So in short: make sure you are using mod_perl, preload the stuff and it will not cost much.