A module is loaded at compile time, no matter where you place the use Module (except in an eval STRING). A module cannot be 'unloaded', although those that specify an unimport may allow you to turn off any semantics that they export using no Module. If you want a module to be loaded only on first demand, you could require it instead.
I personally prefer to put all my use statements at the top of the file (so I know which modules my code needs to run in a single glance), although others may prefer to put them in the subroutines that actually need their functions/semantics.
I wasn't expecting this though:
print Dumper [4,5,6];
print Dumper [1,2,3];
print Dumper [7,8,9];
$VAR1 = [1,2,3];
print() on unopened filehandle Dumper at D:\perl\t.pl line 4.
$VAR1 = [7,8,9];
Why isn't the third print Dumper 'correctly misinterpreted'?