Hi.

I recently wanted to load another version of a module from a module of the same name. i.e. I wanted to load "../ModX.pm" instead of "./ModX.pm" (changing the load path wasn't an option at the time).

I tried this (in ./ModX.pm):

BEGIN {
  delete $INC{'ModX.pm'};
  use lib '../';
  use ModX;
}
[download]

but this just kept the loaded version (./ModX.pm) loaded. After trying a few things, I found this worked:

BEGIN {
  delete $INC{'ModX.pm'};
  use lib '../';
  require '../ModX'; # Yes, I know I haven't done import
}
[download]

Which I found odd, because perldoc -f use says that use is exactly equivalent to: BEGIN { require Module; import Module LIST; } To some extent I was happy, but I was still curious.

Some more playing around and I reaslised my error (which is probably obvious to many, but not to some, hence this post).

use is exactly equivalent to the above, and the issue is the nested BEGIN blocks. The effective code being run (ignoring imports) is:

BEGIN {
  delete $INC{'ModX.pm'};
  use lib '../';
  BEGIN {
    require 'ModX.pm';
  }
}
[download]

and so...the inner BEGIN block executes before the contents of the enclosing block, so you get the effect of something like:

BEGIN {
  require 'ModX.pm';
  delete $INC{'ModX.pm'};
  use lib '../';
}
[download]

where the require does nothing (because we have not yet frobbed %INC).

I thought this interesting. My error was in thinking of 'compile time' versus 'run time'. Everything I was doing was at 'compile time' so I wasn't thinking in terms of that.

Does anyone have anything to offer regarding:

• A better way to do this (other than changing include paths in the environment/command line)
• A way to also perform the 'import' stage of the use?
• Any hidden problems with this approach?

Thanks for reading :-)

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