Where in perlre, the documentation for Perl Regular Expressions did you find that \< means "start of word"? I only find one mention of \<, and that's in the sentence:

So anything that looks like \\, \(, \), \<, \>, \{, or \} is always interpreted as a literal character, not a metacharacter.

.. and that seems to me that it specifically rules out that \< has any meaning beyond a literal <.