* isn't a "wildcard" for Perl regular expressions. Your regex is saying "match and capture 'the' plus zero or more 'r' characters". Consequently, the first 'the' matches.

Try using s/(ther.*)\b/......., for example. "Dot" (.) is the wildcard metacharacter; it matches anything except for a newline, unless you ask Perl to allow it to match the newline too.

Update: Just in case anyone stumbles across this thread while searching for an answer:

Reading perlrequick is a great quick-start to regular expression syntax. Follow it up with perlretut, and then perlre. Additional useful resources include perlop, perlvar, perlrecharclass, perluniprops. But perlrequick is about a half hour of reading for a 70% better understanding of regexes. Each additional document adds a few extra percentage points of knowledge to the foundation that perlrequick builds. ...and of course for the 1000% bonus, "Mastering Regular Expressions" (O'Reilly).