Does $dir contain only "acm"? What does your regex look like in the code? What do you think it looks like when $dir is expanded?

I might write code something like this:

my $dir = "/acm";

#  open file

while (<LOG>) {
    next unless /^\Q$dir\E/;
    # do something with $_ because it matches
}
[download]

If you slurp all of the lines into an array, you could also use the grep command.