if ($out =~/$last(.*)$/s

##</code><code>##

   //m 
   //s 
   //ms 

##</code><code>##

  1 #!/usr/bin/env perl
  2
  3 my $fileA = 'fileA.txt';   # the file to store incremental greps on fileB
  4 my $fileB = 'fileB.txt';   # the live file which is log-rotated every 10MB
  5 my $pattern = 'xxxx';
  6
  7 #--------------------------------
  8 sub main {
  9     $out = qx/grep -A 1 -B 1 $pattern $fileB/;
 10     $out && writeA (\$out);
 11 }
 12 #--------------------------------
 13 sub write_file {
 14     my ($file_name, $content_ref, $write_mod_append) = @_;
 15     my $write_mod = $write_mod_append ? '>>' : '>';
 16     open(my $fh, $write_mod, $file_name) or die "Could not open file '$file_name' $!";
 17     print $fh $$content_ref;
 18     close $fh;
 19 }
 20 #--------------------------------
 21 sub writeA {
 22     my ($out_ref) = @_;
 23     my $write_mod_append;
 24     if ( -e $fileA ) {
 25         $write_mod_append = 1;
 26         my $last_line = qx/tail -1 $fileA/;
 27         chomp $last_line;
 28
 29         if ($$out_ref =~/$last_line(.*)$/s) {
 30             $$out_ref = $1
 31         }
 32     }
 33     write_file ($fileA, $out_ref, $write_mod_append) if ($$out_ref =~ /\S/);
 34 }
 35 #--------------------------------
 36 main;

##</code><code>##

--------------...------------
--------------...------------
--------------aaa------------
-------------xxxx------------
--------------bbb------------
--------------...------------
--------------...------------
--------------ccc------------
-------------xxxx------------
--------------ddd------------
--------------...------------

##</code><code>##

--------------aaa------------
-------------xxxx------------
--------------bbb------------
--
--
--------------ccc------------
-------------xxxx------------
--------------ddd------------

##</code><code>##

--------------...------------
--------------...------------
--------------aaa------------
-------------xxxx------------
--------------bbb------------
--------------...------------
--------------...------------
--------------ccc------------
-------------xxxx------------
--------------ddd------------
--------------...------------
--------------...------------
--------------eee------------
-------------xxxx------------
--------------fff------------
--------------...------------

##</code><code>##

--------------aaa------------
-------------xxxx------------
--------------bbb------------
--
--
--------------ccc------------
-------------xxxx------------
--------------ddd------------
--
--
--------------eee------------
-------------xxxx------------
--------------fff------------