It doesn't matter. What happens is:

1. Some other module or program uses your module.
2. During its compilation, the perl compiler encounters "use Module1";
3. This triggers your module to get compiled; while doing so, Module2 and Module3 will get compiled (if not already compiled). Module2->import and Module3->import will be called.
4. Your module will run. First thing it does is to compile Exporter (if it hasn't been compiled already).
5. Then Module1->import is called (assuming that other module or program uses "use Module1"). Which means Exporter::import is called.
6. Exporter::import exports fn1 and fn2 to the calling module or program.
7. Perl will resume compiling the calling module or program.

It doesn't matter where you place 'require Exporter'.

I've never understood why people use the idiom 'require Exporter'. I never do. 'use Exporter();' also compiles Exporter without calling import.