Haukex and shmem already gave you the right answer, but I think it might be of interest to explain what happens

for my $meth (qw/ WriteLine Select Add /) {
    my $sub = sub {
        my $self = shift;
        my $it = $self->OStream->$meth($_[0]);
        $self->Trace(_CallerStr($it));
        return $it;
    };
    no strict 'refs';
    *$meth = $sub;
}
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In this loop $meth and $sub form a so called closure, each instance of $sub is "closing over" a new instance of the lexical var $meth with a string.

This my $it = $self->OStream->$meth($_[0]); takes advantage of the fact that method calls are (contrary to sub calls) late evaluated strings, i.e. resolved at runtime. (for completeness $meth could also be a code_ref but that's not relevant here)

This

    no strict 'refs';
    *$meth = $sub;
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is assigning a code_ref in $sub to the symbol named in $meth via a so called *glob.

That's how the STASH (Symbol Table hASH) works, each varname in a package has a hash key and the value is a "typeglob" with 6 so called "slots". Assigning a code_ref fills the CODE-slot, you can also address other slots like SCALAR, HASH or ARRAY and so on.

This case showed how to fill the %main:: stash, other namespaces need to be provided explicitly.

no strict 'refs' was necessary because using symbolic references like in *$meth (i.e. $meth is a string not a pointer like reference) is forbidden under strict.

This would work right away ...

*pack::name = sub {};

... but your name has to be dynamic.