The dot . stands for a single character. Use it instead of the *.

See perlretut and perlre for more details.

* 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).

Tip #9 from the Basic debugging checklist : YAPE::Regex::Explain

The regular expression:

(?-imsx:(ther*))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    the                      'the'
----------------------------------------------------------------------
    r*                       'r' (0 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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Use . instead. Also, with different delimiters, you can clean up the replacement string:

use warnings;
use strict;

my $test = "Here is the there";
$test =~ s{(ther.)}{<test>$1</test>}ig;
print "$test\n";

__END__

Here is the <test>there</test>
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