It looks like the $1 is being lost because of the g modifier. That is, it matches, it replaces, it tries again. It starts the matching process again, finds the start of a possible match at the word "in", wipes out $1 etc, finds that the possible match isn't really, and then when the substitution loop finishes, you have lost $1.

This appears to be a bug. (Report with perlbug if you like.) However I would also point out that any code which relies on the correct behaviour is likely to be buggy anyways - if the word appears multiple times then you won't catch all of the substitutions. If you really want to have fine access to all of the substitution information after the fact then you either need to write your own substitution loop (using matching with /g, pos and substr) or you need to embed code in the substitution. Like this:

my @matches;
$line =~ s/\b($word(?:s|ed))/
  push @matches, $1;
  "<b>$1<\/b>"
/iegm;
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