To just get the first occurrence, stop searching after the first one is seen.

foreach my $line (@lines) {
 if ($line=~ m/<FILENAME>.*\.htm/) {
     $doc_title_temp=substr $line, 10;
     $doc_title=$doc_title_temp;
     print "Filename is $doc_title";
     last;  ############### this stops
 };
[download]

With small re-write:

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.htm\n",
             "<FILENAME>dp198076_exfilingfees.htm\n");

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

__END__
Filename is dp198076_424b2-us2342673.htm
[download]