$ perl -we 'q{xxx.xxx} =~ /\b(xxx\.xxx)\b/; print "|$1|\n"'
|xxx.xxx|
$ perl -we 'q{.xxx} =~ /\b(\.xxx)\b/; print "|$1|\n"'
Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
||

##</code><code>##

$ perl -we 'q{xxx.xxx} =~ /(?<!\w)(xxx\.xxx)(?!\w)/; print "|$1|\n"'
|xxx.xxx|
$ perl -we 'q{.xxx} =~ /(?<!\w)(\.xxx)(?!\w)/; print "|$1|\n"'
|.xxx|

##</code><code>##

$ cat log_to_read.txt
1 abc
2 def
3 ghi
4 jkl
5 def
6 ghi
7 abd
8 abcdefghi
9 abc def ghi
10 $@%
11 \$\@\%

$ cat keys_to_find.txt
abc
def
ghi
$@%

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

my $log_search_re;

{
    open my $keys_fh, '<', 'keys_to_find.txt';
    $log_search_re = qr/(?x: (?<!\w) (?:
        @{[ join '|', map { chomp; "\Q$_" } <$keys_fh> ]}
    ) (?!\w) )/;
}

{
    open my $log_fh, '<', 'log_to_read.txt';
    open my $out_fh, '>', 'log_lines_matched.txt';
    /$log_search_re/ && print $out_fh $_ while <$log_fh>;
}

##</code><code>##

$ cat log_lines_matched.txt
1 abc
2 def
3 ghi
5 def
6 ghi
9 abc def ghi
10 $@%