What does $1 mean in Perl?

Modules have to return a true value. 1 is a true value.

When you load a module "Foo" with use Foo or require(), perl executes the Foo.pm file like an ordinary script. It expects it to return a true value if the module was loaded correctly. The 1; does that. It could be 2; or "hey there"; just as well.

The block around the declaration of $somevar and the function Somesub limits the scope of the variable. That way, it is only accessible from Somesub and doesn't get cleared on each invocation of Somesub (which would be the case if it was declared inside the function body). This idiom has been superseded in recent versions of perl (5.10 and up) which have the state keyword.

1 at the end of a module means that the module returns true to use/require statements. It can be used to tell if module initialization is successful. Otherwise, use/require will fail.

$somevar is a variable which is accessable only inside the block. It is used to simulate "static" variables. Starting from Perl 5.10 you can use keyword state keyword to have the same results:

## Starting from Perl 5.10 you can specify "static" variables directly.
sub Somesub {
    state $somevar;
}