my $str = "This is one thing. This.";

sub cref_regex {
    my ($str, $re_cref) = @_;
    print $re_cref->($str);
}

my $cref = sub { $_[0] =~ s/This/That/ig; };

cref_regex($str, $cref);

##</code><code>##

sub separate_regex {
    my ($str, $search, $replace) = @_;

    if (! $replace){
        print $str =~ /$search/;
    }
    else {
        print $str =~ s/$search/$replace/g;
    }
}

separate_regex($str, 'This', 'That');