my $fn = shift;
exit 1 if not defined $fn;

my $input = do {
    open my $fh, "<", $fn or die "open failed";
    local $/;
    <$fh>
};

my $count = () = $input =~ m/mul\(\d{1,3},\d{1,3}\)/g;

print "Found $count matches.\n";