Well, basically the error message tries to tell you that you forgot to define your package's constructor. You can name your constructors as you like, naming it new is a mere convention. And it is missing something like

sub new {
    return bless {}, __PACKAGE__;
}
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Of course, you could name your constructors differently, e.g. after defining

sub File {
    return bless {}, 'File';
}
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you would use File->File instead of File->new.