What I'd like to do is redefine what "\b" is.

You can, by overloading regular expressions. The following is just a quick hack I threw together, but I think it catches most cases, including /[\b]/ and /\\b/.

#!/usr/bin/perl

use strict;
use warnings;
no warnings qw /syntax/;

BEGIN {
    my $B = '(?:(?<=[\w/])(?=[^\w/])|'  .
               '(?<=[^\w/])(?=[\w/])|'  .
              '^(?=[\w/])|(?<=[\w/])$)';
    $^H |= 0x30000;
    $^H {qr} = sub {
        local $_ = $_ [1];
        s/(\[\^?]?[^]]*]|[^\\]+|\\.)/$1 eq '\b' ? $B : $1/eg;
        $_;
    }
}


while (<DATA>) {
    chomp;
    print "$_: ";
    print $& if /\b\w+\b/;
    print "\n";
}

__DATA__
foo
&foo&
&foo/bar&


foo: foo
&foo&: foo
&foo/bar&:
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Abigail