By having the linter make the giant assumption that all subs are methods, that tells you a bit about $self, but not enough. It only helps determine that cases such as the following aren't problems:

{   package Base;
    sub x { ... }
}

{   package Child;
    our @ISA = 'Base';
    sub y { shift->x }
}
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However, it doesn't help determine that something is a problem. For example, is there a problem in the following?

package Package;
sub y { shift->x }
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No, turns out package Package is just a role:

{   package SomeRole;
    # Implementor must provide y
    sub y { shift->x }
}

{   package Class;
    our @ISA = 'SomeRole';
    sub x { ... }
}
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But that's not the real test. How often do you call a child method that doesn't exist? I expect the majority of problems to be in code such as the following:

sub some_method {
    my ($self, $arg) = @_
    ...
    $arg->other_method();
    ...
}
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or

sub some_method {
    my ($self) = @_
    ...
    $self->attribute->other_method();
    ...
}
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The linter knows absolutely nothing about the class or object whose method other_method is being called, so it has no idea if other_method exists for that class or object.