The penultimate line of README.md is:

https://www.perlmonks.org/?node\_id=155288 for the getopt/usage model.
[download]

The backslash in the URL is rendered as is (i.e. https://www.perlmonks.org/?node\_id=155288); the link just goes to "The Monastery Gates".

I use GitLab a lot, but not GitHub. I suspect that you don't need the backslash in the URL (but would in any link text had that been included): I don't have any easy way to test this; you may find https://github.github.com/gfm/#links useful.