With the scope resolution operator, you get a normal subroutine call. But with the dereferencing operator, you get a method call in which the name of the class is passed implicitly as the first argument. So

Foo::bar(1, 2, 3);
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calls sub bar in the Foo package and passes 1, 2, 3 as the arguments (which are aliased in @_). But

Foo->bar(1, 2, 3);
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calls Foo::bar with the arguments Foo, 1, 2, 3. Likewise, if $obj is of class Foo, then

$obj->bar(1, 2, 3);
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calls Foo::bar with the arguments Foo, 1, 2, 3.

See Methods in perlootut and Method Invocation in perlobj.

Hope that helps,