$ head File[AB].log
==> FileA.log <==
Application, This occured blah
Application, That occured blah
Application, Also this
AnotherLog, Bob did this
AnotherLog, Dave did that

==> FileB.log <==
Uk, London, Application, datetime, LaterDateTime, Today it had'nt occured blah but I dont care
Uk, London, Application, datetime, LaterDateTime, Today This occured blah and I care
USA, Texas, Application, datetime, LaterDateTime, Today This may have occured blah but I dont care
$ cat script.pl
use warnings;
use strict;

die qq[Usage: perl $0 <fileA> <fileB>\n] unless @ARGV == 2;

my $file_in_process = 1;
my %fileA;

while ( <> ) {
        next if m/\A\s*\Z/;
        chomp;
        my @f = split /\s*,\s*/;

        if ( $file_in_process == 1 ) {
                next unless @f == 2;
                push @{ $fileA{ $f[0] } }, $f[1];
                next;
        }

        if ( exists $fileA{ $f[2] } and grep { $f[5] =~ m/$_/ } @{ $fileA{ $f[2] } } ) {
                printf qq[%s\n], $_;
        }
}
continue {
        if ( eof ) {
                ++$file_in_process;
        }
}
$ perl script.pl File[AB].log
Uk, London, Application, datetime, LaterDateTime, Today This occured blah and I care