This might actually be explorer messing up the interpolation, since echo http://www.perlmonks.com/?node_id=816742 behaves as expected. Could not find any web resources, however, describing how explorer parses CLI arguments.

Naaaah, that would be too easy. echo is an internal command, and thus gets a different set of parsing rules. Both command.com and cmd.exe have a long history of really smart people finding useful bugs in the parser, and newer versions have to be bug-compatible to allow running old batches. MS explains the various required rules for quoting arguments somewhere in their knowledge base. But compared to a modern unix shell, both command.com and cmd.exe just cause a lot of pain. I avoid them at any cost, usually by using Perl or by switching to Linux.

Alexander