Your code in sub sub2 ... is calling _sub1() without arguments. It should probably look more like:

    my $self = shift;
    return $self->_sub1();
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I'll also point out that prototypes are meaningless when used with OO methods: see perlsub - Prototypes. I'd suggest changing all instances of:

sub XXX() { ... }
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to

sub XXX { ... }
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