You should read the documentations of Inline itself:

In case you've forgotten, the DATA pseudo file is comprised of all the text after the __END__ or __DATA__ section of your program. If you're working outside the main package, you'd best use the __DATA__ marker or else Inline will not find your code.

perldata might also be of interest:

For compatibility with older scripts written before __DATA__ was introduced, __END__ behaves like __DATA__ in the top level script (but not in files loaded with "require" or "do") and leaves the remaining contents of the file accessible via "main::DATA".

use says:

use Module LIST... is exactly equivalent to

    BEGIN { require Module; Module->import( LIST ); }
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except that Module must be a bareword.