use POSIX qw(strftime);

sub log_msg {
  open(my $fh, ">>", 'log_file.txt') or die "Err: $!";
  printf $fh "[%s] %s\n", strftime("%Y-%m-%d %H:%M:%S", localtime), shift;
  close $fh;
}

...

log_msg("Log this message");