my $find_re = qr/1\.2\.3/i;
...
grep /$find_re/, ...
...

##</code><code>##

my $find_str = '1.2.3';
my $find_re  = quotemeta($find_str);
...
grep /$find_re/i, ...
...

##</code><code>##

my $find_str = '1.2.3';
...
grep /\Q$find_str\E/i, ...
...