This is an old question, but for anyone else looking for the answer, most licenses don't require the license to be included *inside* each source file, as long as the license is included *with* the source code. Often this is done by use of a `LICENSE.md` or `LICENSE.txt` file included in the same repo, folder, or directory as the code.

Below is a rundown of some common open source licenses and their requirements regarding inclusion in source files.

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##Apache License Version 2.0

One of the (many) reasons Apache released v2.0 of the [Apache License](https://www.apache.org/licenses/) was:

>... to allow the license to be included by reference instead of listed in every file... ([source](http://mail-archives.apache.org/mod_mbox/www-announce/200401.mbox/%[email protected]%3E))

The license's [Appendix](https://www.apache.org/licenses/LICENSE-2.0#apply) seems to specify that a license notice needs to be included in the file itself, but GitHub's [ChooseALicense.com](https://choosealicense.com/licenses/apache-2.0/) says that this requirement is more of a recommendation.

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##MIT License

The [MIT License](https://mit-license.org/) only requires that the license text:

>be included in all copies or substantial portions of the Software.

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##GPL / LGPL / AGPL

For the GPL, LGPL, or AGPL license, the Free Software Foundation,
 which runs [GNU.org](https://www.gnu.org/licenses/gpl-howto.html), *does* want a notice included within each file (as well as in an accompanying `COPYING` file containing the full license text, or `COPYING LESSER` if using the LGPL).

Here's how they say to include the license in your source files:

>This involves adding two elements to each source file of your program: a copyright notice (such as “Copyright 1999 Terry Jones”), and a statement of copying permission, saying that the program is distributed under the terms of the GNU General Public License (or the Lesser GPL, or the Affero GPL).

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*Note that I am not a lawyer and this answer does not even remotely constitute any sort of legal advice.*