# script.pl
use Foo;
my $foo = Foo->new();
$foo->do();

# does:

1.Compile script.pl
  1.Compile use Foo;
  2.Execute require Foo;
    1.Compile Foo.pm
      1. ...
    2.Execute Foo.pm
      1. ...
  3.Execute import Foo;
  4.Compile my $foo = Foo->new();
  5.Compile $foo->do();
2.Execute script.pl
  1.Execute my $foo = Foo->new();
  2.Execute $foo->do();