Here's the line, explained a bit:

my $regex = join '|', map "\Q$_\E?", @filelist;
                  ^         ^ ^ ^^
                  |         | | ||
                  |         | | |+ - Last character in file name optio
+nal, i.e., the "l" in html
                  |         | | + - End quote metacharacters
                  |         | + - The default scalar aliased to each @
+filelist element
                  |         + - Begin quote metacharacters (e.g., the 
+period)
                  + - Join each element with alternation ("or") charac
+ter
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As shown, this results in the following:

1234567_3a_20101000\.html?|99877_b_20111111\.html?|99877_c_20111111\.h
+tml? ...
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This line builds a regex using the file names in @filelist. When passed to File::Find::Rule as the -name rule, only those files names which match the regex will be returned by File::Find::Rule, if any.

If you want File::Find::Rule to look for a particular pattern, change the regex here:

->name(qr/^(?:$regex)$/)
          ^^^^^^^^^^^^
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However, if you want to process the files returned by File::Find::Rule, you can do something like this:

my @matchingFileNames = grep /pattern/, @found_html;
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where "pattern" represents the regex that would 'filter' the elements of @found_html.