To help us communicate without going around in circles, please read and try to follow:

• I know what I mean. Why don't you?
• Short, Self-Contained, Correct Example

Here is my test program t1.pl:

use strict;
use warnings;

# Small standalone test program derived from [id://11153804]
# @lines contains some test lines derived from:
# https://www.sec.gov/Archives/edgar/data/831001/000095010323011632/00
+00950103-23-011632.txt
my @lines = (
   '<FILENAME>dp198076_424b2-us2342673.htm',
   '<FILENAME>dp198076_exfilingfees.htm',
   '<FILENAME>dp198076_oopswrongextension.htmfred',
);

foreach my $line (@lines) {
   print "line:$line:\n";
   if ($line =~ m/<FILENAME>(.*\.htm)/) {
      print "   matched line: filename='$1'\n";
   }
   else {
      print "   did not match line\n";
   }
}
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When I run this program, this is what I see:

line:<FILENAME>dp198076_424b2-us2342673.htm:
   matched line: filename='dp198076_424b2-us2342673.htm'
line:<FILENAME>dp198076_exfilingfees.htm:
   matched line: filename='dp198076_exfilingfees.htm'
line:<FILENAME>dp198076_oopswrongextension.htmfred:
   matched line: filename='dp198076_oopswrongextension.htm'
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When I change this line above from:

   if ($line =~ m/<FILENAME>(.*\.htm)/) {
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   if ($line =~ m/<FILENAME>(.*\.htm)\b/) {
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when I run this program I see instead:

line:<FILENAME>dp198076_424b2-us2342673.htm:
   matched line: filename='dp198076_424b2-us2342673.htm'
line:<FILENAME>dp198076_exfilingfees.htm:
   matched line: filename='dp198076_exfilingfees.htm'
line:<FILENAME>dp198076_oopswrongextension.htmfred:
   did not match line
[download]

If that is not what you were asking about, please clarify your question by posting a Short, Self-Contained, Correct Example that we can run.

use strict;
use warnings;

my @lines = ("https://www.sec.gov/Archives/edgar/data/831001/000095010
+323011632/0000950103-23-011632.txt\n",
             "<FILENAME>dp198076_424b2-us2342673.htmSomeCrap\n",
             "<FILENAME>dp198076_424b2-us2342673.htm\n",
             "<FILENAME>dp198076_exfilingfees.htm\n",
            );

foreach my $line (@lines) {
  if (my ($doc_title) = $line=~ m/<FILENAME>(.*\.htm)\b/) {    
     print "Filename is $doc_title\n";
     last;  ############### this stops
     }
}

__END__
Filename is dp198076_424b2-us2342673.htm
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You were not that far off...

The file is actually this:

"<FILENAME>dp198076_424b2-us2342673.htm \n", "<FILENAME>dp198076_exfilingfees.htm\n",

with a space after the .htm in some cases but not others so the /b didn't work all the time

You will have to show some runnable code where the \b fails. Both of your example lines work fine in my example code.

\b means approximately "word boundary". Any white space character (space or \n or other such character like \t) satisfies that boundary condition. End of the string also satisfies that boundary condition (i.e. having no character following ".htm").

What do you mean by " so the /b didn't work all the time"?

Look carefully and make sure that there is no space before the \b in:
if (my ($doc_title) = $line=~ m/<FILENAME>(.*\.htm)\b/) {