Compile: use Foo;
Compile: BEGIN { require Foo; import Foo; }
Run:     require Foo;
         (starts compiling Foo.pm)
Compile: my $shell = $ENV{'SHELL'} || "";
         (my $shell; gets declared)
Compile: BEGIN { ... }
Run:     Code from above BEGIN block
         ($shell is declared but uninitialized here)
Compile: 1;
         (Foo.pm fully compiled, start running it)
Run:     my $shell = $ENV{'SHELL'} || "";
Run:     1;
         (require Foo; returns)
Run:     import Foo;
...